What is Type-safe

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What does  type-safe  mean

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Type-safe means that programmatically  the type of data for a variable  return value  or argument must fit within a certain criteria   In practice  this means that 7  an integer type  is different from  7   a quoted character of string type    PHP  Javascript and other dynamic scripting languages are usually weakly-typed  in that they will convert a  string   7  to an  integer  7 if you try to add  7    3  although sometimes you have to do this explicitly  and Javascript uses the     character for concatenation    C C   Java will not understand that  or will concatenate the result into  73  instead  Type-safety prevents these types of bugs in code by making the type requirement explicit   Type-safety is very useful  The solution to the above  7    3 would be to type cast  int   7    3  equals 10

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Try this explanation on     TypeSafe means that variables are statically checked for appropriate assignment at compile time  For example  consder a string or an integer  These two different data types cannot be cross-assigned  ie  you can t assign an integer to a string nor can you assign a string to an integer    For non-typesafe behavior  consider this   object x   89  int y    if you attempt to do this   y   x    the compiler throws an error that says it can t convert a System Object to an Integer  You need to do that explicitly  One way would be   y   Convert ToInt32  x      The assignment above is not typesafe  A typesafe assignement is where the types can directly be assigned to each other    Non typesafe collections abound in ASP NET  eg  the application  session  and viewstate collections   The good news about these collections is that  minimizing multiple server state management considerations  you can put pretty much any data type in any of the three collections  The bad news  because these collections aren t typesafe  you ll need to cast the values appropriately when you fetch them back out    For example   Session   x      34    works fine  But to assign the integer value back  you ll need to   int i   Convert ToInt32  Session   x         Read about generics for ways that facility helps you easily implement typesafe collections    C  is a typesafe language but watch for articles about C  4 0  interesting dynamic possibilities loom  is it a good thing that C  is essentially getting Option Strict  Off    we ll see

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Try this explanation on     TypeSafe means that variables are statically checked for appropriate assignment at compile time  For example  consder a string or an integer  These two different data types cannot be cross-assigned  ie  you can t assign an integer to a string nor can you assign a string to an integer    For non-typesafe behavior  consider this   object x   89  int y    if you attempt to do this   y   x    the compiler throws an error that says it can t convert a System Object to an Integer  You need to do that explicitly  One way would be   y   Convert ToInt32  x      The assignment above is not typesafe  A typesafe assignement is where the types can directly be assigned to each other    Non typesafe collections abound in ASP NET  eg  the application  session  and viewstate collections   The good news about these collections is that  minimizing multiple server state management considerations  you can put pretty much any data type in any of the three collections  The bad news  because these collections aren t typesafe  you ll need to cast the values appropriately when you fetch them back out    For example   Session   x      34    works fine  But to assign the integer value back  you ll need to   int i   Convert ToInt32  Session   x         Read about generics for ways that facility helps you easily implement typesafe collections    C  is a typesafe language but watch for articles about C  4 0  interesting dynamic possibilities loom  is it a good thing that C  is essentially getting Option Strict  Off    we ll see

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Many answers here conflate type-safety with static-typing and dynamic-typing  A dynamically typed language  like smalltalk  can be type-safe as well   A short answer  a language is considered type-safe if no operation leads to undefined behavior  Many consider the requirement of explicit type conversions necessary for a language to be strictly typed  as automatic conversions can sometimes leads to well defined but unexpected unintuitive behaviors

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Type-safe means that the set of values that may be assigned to a program variable must fit well-defined and testable criteria  Type-safe variables lead to more robust programs because the algorithms that manipulate the variables can trust that the variable will only take one of a well-defined set of values  Keeping this trust ensures the integrity and quality of the data and the program    For many variables  the set of values that may be assigned to a variable is defined at the time the program is written  For example  a variable called  colour  may be allowed to take on the values  red    green   or  blue  and never any other values  For other variables those criteria may change at run-time  For example  a variable called  colour  may only be allowed to take on values in the  name  column of a  Colours  table in a relational database  where  red   green   and  blue   are three values for  name  in the  Colours  table  but some other part of the computer program may be able to add to that list while the program is running  and the variable can take on the new values after they are added to the Colours table    Many type-safe languages give the illusion of  type-safety  by insisting on strictly defining types for variables and only allowing a variable to be assigned values of the same  type   There are a couple of problems with this approach  For example  a program may have a variable  yearOfBirth  which is the year a person was born  and it is tempting to type-cast it as a short integer  However  it is not a short integer  This year  it is a number that is less than 2009 and greater than -10000  However  this set grows by 1 every year as the program runs  Making this a  short int  is not adequate  What is needed to make this variable type-safe is a run-time validation function that ensures that the number is always greater than -10000 and less than the next calendar year  There is no compiler that can enforce such criteria because these criteria are always unique characteristics of the problem domain    Languages that use dynamic typing  or duck-typing  or manifest typing  such as Perl  Python  Ruby  SQLite  and Lua don t have the notion of typed variables  This forces the programmer to write a run-time validation routine for every variable to ensure that it is correct  or endure the consequences of unexplained run-time exceptions  In my experience  programmers in statically typed languages such as C  C    Java  and C  are often lulled into thinking that statically defined types is all they need to do to get the benefits of type-safety  This is simply not true for many useful computer programs  and it is hard to predict if it is true for any particular computer program    The long  amp  the short     Do you want type-safety  If so  then write run-time functions to ensure that when a variable is assigned a value  it conforms to well-defined criteria  The down-side is that it makes domain analysis really difficult for most computer programs because you have to explicitly define the criteria for each program variable

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To get a better understanding do watch the below video which demonstrates code in type safe language  C   and NOT type safe language   javascript    http   www youtube com watch v Rlw njQhkxw  Now for the long text   Type safety means preventing type errors  Type error occurs when data type of one type is assigned to other type UNKNOWINGLY and we get undesirable results   For instance JavaScript is a NOT a type safe language  In the below code    num    is a numeric variable and    str    is string   Javascript  allows me to do    num   str      now GUESS will it do arithmetic or concatenation    Now for the below code the results are    55    but the important point is the confusion created what kind of operation it will do    This is happening because javascript is not a type safe language  Its allowing to set one type of data to the other type without restrictions    lt script gt  var num   5     numeric var str    5      string var z   num   str     arthimetic or concat      alert z      displays     55     lt  script gt    C  is a type safe language   It does not allow one data type to be assigned to other data type  The below code does not allow         operator on different data types

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Type safety is not just a compile time constraint  but a run time constraint  I feel even after all this time  we can add further clarity to this    There are 2 main issues related to type safety  Memory   and data type  with its corresponding operations    Memory    A char typically requires 1 byte per character  or 8 bits  depends on language  Java and C  store unicode chars which require 16 bits   An int requires 4 bytes  or 32 bits  usually    Visually   char   - - - - - - - -   int    - - - - - - - -   - - - - - - - -   - - - - - - - -   - - - - - - - -   A type safe language does not allow an int to be inserted into a char at run-time  this should throw some kind of class cast or out of memory exception   However  in a type unsafe language  you would overwrite existing data in 3 more adjacent bytes of memory   int  gt  gt  char    - - - - - - - -                                                         In the above case  the 3 bytes to the right are overwritten  so any pointers to that memory  say 3 consecutive chars  which expect to get a predictable char value will now have garbage  This causes undefined behavior in your program  or worse  possibly in other programs depending on how the OS allocates memory - very unlikely these days        While this first issue is not technically about data type  type safe languages address it inherently and it visually describes the issue to those unaware of how memory allocation  looks    Data Type  The more subtle and direct type issue is where two data types use the same memory allocation  Take a int vs an unsigned int  Both are 32 bits   Just as easily could be a char 4  and an int  but the more common issue is uint vs  int     - - - - - - - -   - - - - - - - -   - - - - - - - -   - - - - - - - -    - - - - - - - -   - - - - - - - -   - - - - - - - -   - - - - - - - -   A type unsafe language allows the programmer to reference a properly allocated span of 32 bits  but when the value of a unsigned int is read into the space of an int  or vice versa   we again have undefined behavior  Imagine the problems this could cause in a banking program        Dude  I overdrafted  30 and now I have  65 506 left          course  banking programs use much larger data types     LOL   As others have already pointed out  the next issue is computational operations on types  That has already been sufficiently covered   Speed vs Safety  Most programmers today never need to worry about such things unless they are using something like C or C    Both of these languages allow programmers to easily violate type safety at run time  direct memory referencing  despite the compilers  best efforts to minimize the risk  HOWEVER  this is not all bad    One reason these languages are so computationally fast is they are not burdened by verifying type compatibility during run time operations like  for example  Java  They assume the developer is a good rational being who won t add a string and an int together and for that  the developer is rewarded with speed efficiency

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To get a better understanding do watch the below video which demonstrates code in type safe language  C   and NOT type safe language   javascript    http   www youtube com watch v Rlw njQhkxw  Now for the long text   Type safety means preventing type errors  Type error occurs when data type of one type is assigned to other type UNKNOWINGLY and we get undesirable results   For instance JavaScript is a NOT a type safe language  In the below code    num    is a numeric variable and    str    is string   Javascript  allows me to do    num   str      now GUESS will it do arithmetic or concatenation    Now for the below code the results are    55    but the important point is the confusion created what kind of operation it will do    This is happening because javascript is not a type safe language  Its allowing to set one type of data to the other type without restrictions    lt script gt  var num   5     numeric var str    5      string var z   num   str     arthimetic or concat      alert z      displays     55     lt  script gt    C  is a type safe language   It does not allow one data type to be assigned to other data type  The below code does not allow         operator on different data types

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Type-safe means that the set of values that may be assigned to a program variable must fit well-defined and testable criteria  Type-safe variables lead to more robust programs because the algorithms that manipulate the variables can trust that the variable will only take one of a well-defined set of values  Keeping this trust ensures the integrity and quality of the data and the program    For many variables  the set of values that may be assigned to a variable is defined at the time the program is written  For example  a variable called  colour  may be allowed to take on the values  red    green   or  blue  and never any other values  For other variables those criteria may change at run-time  For example  a variable called  colour  may only be allowed to take on values in the  name  column of a  Colours  table in a relational database  where  red   green   and  blue   are three values for  name  in the  Colours  table  but some other part of the computer program may be able to add to that list while the program is running  and the variable can take on the new values after they are added to the Colours table    Many type-safe languages give the illusion of  type-safety  by insisting on strictly defining types for variables and only allowing a variable to be assigned values of the same  type   There are a couple of problems with this approach  For example  a program may have a variable  yearOfBirth  which is the year a person was born  and it is tempting to type-cast it as a short integer  However  it is not a short integer  This year  it is a number that is less than 2009 and greater than -10000  However  this set grows by 1 every year as the program runs  Making this a  short int  is not adequate  What is needed to make this variable type-safe is a run-time validation function that ensures that the number is always greater than -10000 and less than the next calendar year  There is no compiler that can enforce such criteria because these criteria are always unique characteristics of the problem domain    Languages that use dynamic typing  or duck-typing  or manifest typing  such as Perl  Python  Ruby  SQLite  and Lua don t have the notion of typed variables  This forces the programmer to write a run-time validation routine for every variable to ensure that it is correct  or endure the consequences of unexplained run-time exceptions  In my experience  programmers in statically typed languages such as C  C    Java  and C  are often lulled into thinking that statically defined types is all they need to do to get the benefits of type-safety  This is simply not true for many useful computer programs  and it is hard to predict if it is true for any particular computer program    The long  amp  the short     Do you want type-safety  If so  then write run-time functions to ensure that when a variable is assigned a value  it conforms to well-defined criteria  The down-side is that it makes domain analysis really difficult for most computer programs because you have to explicitly define the criteria for each program variable

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An explanation from a liberal arts major  not a comp sci major   When people say that a language or language feature is type safe  they mean that the language will help prevent you from  for example  passing something that isn t an integer to some logic that expects an integer   For example  in C   I define a function as    void foo int arg    The compiler will then stop me from doing this          call foo   foo  hello world     In other languages  the compiler would not stop me  or there is no compiler      so the string would be passed to the logic and then probably something bad will happen   Type safe languages try to catch more at  compile time    On the down side  with type safe languages  when you have a string like  123  and you want to operate on it like an int  you have to write more code to convert the string to an int  or when you have an int like 123 and want to use it in a message like   The answer is 123   you have to write more code to convert cast it to a string

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An explanation from a liberal arts major  not a comp sci major   When people say that a language or language feature is type safe  they mean that the language will help prevent you from  for example  passing something that isn t an integer to some logic that expects an integer   For example  in C   I define a function as    void foo int arg    The compiler will then stop me from doing this          call foo   foo  hello world     In other languages  the compiler would not stop me  or there is no compiler      so the string would be passed to the logic and then probably something bad will happen   Type safe languages try to catch more at  compile time    On the down side  with type safe languages  when you have a string like  123  and you want to operate on it like an int  you have to write more code to convert the string to an int  or when you have an int like 123 and want to use it in a message like   The answer is 123   you have to write more code to convert cast it to a string

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Type-safe means that programmatically  the type of data for a variable  return value  or argument must fit within a certain criteria   In practice  this means that 7  an integer type  is different from  7   a quoted character of string type    PHP  Javascript and other dynamic scripting languages are usually weakly-typed  in that they will convert a  string   7  to an  integer  7 if you try to add  7    3  although sometimes you have to do this explicitly  and Javascript uses the     character for concatenation    C C   Java will not understand that  or will concatenate the result into  73  instead  Type-safety prevents these types of bugs in code by making the type requirement explicit   Type-safety is very useful  The solution to the above  7    3 would be to type cast  int   7    3  equals 10

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Many answers here conflate type-safety with static-typing and dynamic-typing  A dynamically typed language  like smalltalk  can be type-safe as well   A short answer  a language is considered type-safe if no operation leads to undefined behavior  Many consider the requirement of explicit type conversions necessary for a language to be strictly typed  as automatic conversions can sometimes leads to well defined but unexpected unintuitive behaviors

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Type safety is not just a compile time constraint  but a run time constraint  I feel even after all this time  we can add further clarity to this    There are 2 main issues related to type safety  Memory   and data type  with its corresponding operations    Memory    A char typically requires 1 byte per character  or 8 bits  depends on language  Java and C  store unicode chars which require 16 bits   An int requires 4 bytes  or 32 bits  usually    Visually   char   - - - - - - - -   int    - - - - - - - -   - - - - - - - -   - - - - - - - -   - - - - - - - -   A type safe language does not allow an int to be inserted into a char at run-time  this should throw some kind of class cast or out of memory exception   However  in a type unsafe language  you would overwrite existing data in 3 more adjacent bytes of memory   int  gt  gt  char    - - - - - - - -                                                         In the above case  the 3 bytes to the right are overwritten  so any pointers to that memory  say 3 consecutive chars  which expect to get a predictable char value will now have garbage  This causes undefined behavior in your program  or worse  possibly in other programs depending on how the OS allocates memory - very unlikely these days        While this first issue is not technically about data type  type safe languages address it inherently and it visually describes the issue to those unaware of how memory allocation  looks    Data Type  The more subtle and direct type issue is where two data types use the same memory allocation  Take a int vs an unsigned int  Both are 32 bits   Just as easily could be a char 4  and an int  but the more common issue is uint vs  int     - - - - - - - -   - - - - - - - -   - - - - - - - -   - - - - - - - -    - - - - - - - -   - - - - - - - -   - - - - - - - -   - - - - - - - -   A type unsafe language allows the programmer to reference a properly allocated span of 32 bits  but when the value of a unsigned int is read into the space of an int  or vice versa   we again have undefined behavior  Imagine the problems this could cause in a banking program        Dude  I overdrafted  30 and now I have  65 506 left          course  banking programs use much larger data types     LOL   As others have already pointed out  the next issue is computational operations on types  That has already been sufficiently covered   Speed vs Safety  Most programmers today never need to worry about such things unless they are using something like C or C    Both of these languages allow programmers to easily violate type safety at run time  direct memory referencing  despite the compilers  best efforts to minimize the risk  HOWEVER  this is not all bad    One reason these languages are so computationally fast is they are not burdened by verifying type compatibility during run time operations like  for example  Java  They assume the developer is a good rational being who won t add a string and an int together and for that  the developer is rewarded with speed efficiency

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Type safety means that the compiler will validate types while compiling  and throw an error if you try to assign the wrong type to a variable   Some simple examples      Fails  Trying to put an integer in a string String one   1     Also fails  int foo    bar     This also applies to method arguments  since you are passing explicit types to them   int AddTwoNumbers int a  int b        return a   b      If I tried to call that using   int Sum   AddTwoNumbers 5   5      The compiler would throw an error  because I am passing a string   5    and it is expecting an integer   In a loosely typed language  such as javascript  I can do the following   function AddTwoNumbers a  b        return a   b      if I call it like this   Sum   AddTwoNumbers 5   5      Javascript automaticly converts the 5 to a string  and returns  55   This is due to javascript using the   sign for string concatenation  To make it type-aware  you would need to do something like   function AddTwoNumbers a  b        return Number a    Number b       Or  possibly   function AddOnlyTwoNumbers a  b        if  isNaN a     isNaN b           return false      return Number a    Number b       if I call it like this   Sum   AddTwoNumbers 5    dogs      Javascript automatically converts the 5 to a string  and appends them  to return  5 dogs    Not all dynamic languages are as forgiving as javascript  In fact a dynamic language does not implicity imply a loose typed language  see Python    some of them will actually give you a runtime error on invalid type casting   While its convenient  it opens you up to a lot of errors that can be easily missed  and only identified by testing the running program  Personally  I prefer to have my compiler tell me if I made that mistake   Now  back to C      C  supports a language feature called covariance  this basically means that you can substitute a base type for a child type and not cause an error  for example    public class Foo   Bar         Here  I created a new class  Foo  that subclasses Bar  I can now create a method    void DoSomething Bar myBar    And call it using either a Foo  or a Bar as an argument  both will work without causing an error  This works because C  knows that any child class of Bar will implement the interface of Bar   However  you cannot do the inverse   void DoSomething Foo myFoo    In this situation  I cannot pass Bar to this method  because the compiler does not know that Bar implements Foo s interface  This is because a child class can  and usually will  be much different than the parent class   Of course  now I ve gone way off the deep end and beyond the scope of the original question  but its all good stuff to know

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Concept   To be very simple Type Safe like the meanings  it makes sure that type of the variable should be safe like    no wrong data type e g  can t save or initialized a variable of string type with integer Out of bound indexes are not accessible Allow only the specific memory location    so it is all about the safety of the types of your storage in terms of variables

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Type-safety should not be confused with static   dynamic typing or strong   weak typing   A type-safe language is one where the only operations that one can execute on data are the ones that are condoned by the data s type  That is  if your data is of type X and X doesn t support operation y  then the language will not allow you to to execute y X    This definition doesn t set rules on when this is checked  It can be at compile time  static typing  or at runtime  dynamic typing   typically through exceptions  It can be a bit of both  some statically typed languages allow you to cast data from one type to another  and the validity of casts must be checked at runtime  imagine that you re trying to cast an Object to a Consumer - the compiler has no way of knowing whether it s acceptable or not    Type-safety does not necessarily mean strongly typed  either - some languages are notoriously weakly typed  but still arguably type safe  Take Javascript  for example  its type system is as weak as they come  but still strictly defined  It allows automatic casting of data  say  strings to ints   but within well defined rules  There is to my knowledge no case where a Javascript program will behave in an undefined fashion  and if you re clever enough  I m not   you should be able to predict what will happen when reading Javascript code   An example of a type-unsafe programming language is C  reading   writing an array value outside of the array s bounds has an undefined behaviour by specification  It s impossible to predict what will happen  C is a language that has a type system  but is not type safe

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Type-Safe is code that accesses only the memory locations it is authorized to access  and only in well-defined  allowable ways   Type-safe code cannot perform an operation on an object that is invalid for that object  The C  and VB NET language compilers always produce type-safe code  which is verified to be type-safe during JIT compilation

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An explanation from a liberal arts major  not a comp sci major   When people say that a language or language feature is type safe  they mean that the language will help prevent you from  for example  passing something that isn t an integer to some logic that expects an integer   For example  in C   I define a function as    void foo int arg    The compiler will then stop me from doing this          call foo   foo  hello world     In other languages  the compiler would not stop me  or there is no compiler      so the string would be passed to the logic and then probably something bad will happen   Type safe languages try to catch more at  compile time    On the down side  with type safe languages  when you have a string like  123  and you want to operate on it like an int  you have to write more code to convert the string to an int  or when you have an int like 123 and want to use it in a message like   The answer is 123   you have to write more code to convert cast it to a string

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Type-Safe is code that accesses only the memory locations it is authorized to access  and only in well-defined  allowable ways   Type-safe code cannot perform an operation on an object that is invalid for that object  The C  and VB NET language compilers always produce type-safe code  which is verified to be type-safe during JIT compilation

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Concept   To be very simple Type Safe like the meanings  it makes sure that type of the variable should be safe like    no wrong data type e g  can t save or initialized a variable of string type with integer Out of bound indexes are not accessible Allow only the specific memory location    so it is all about the safety of the types of your storage in terms of variables

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Type-safe means that programmatically  the type of data for a variable  return value  or argument must fit within a certain criteria   In practice  this means that 7  an integer type  is different from  7   a quoted character of string type    PHP  Javascript and other dynamic scripting languages are usually weakly-typed  in that they will convert a  string   7  to an  integer  7 if you try to add  7    3  although sometimes you have to do this explicitly  and Javascript uses the     character for concatenation    C C   Java will not understand that  or will concatenate the result into  73  instead  Type-safety prevents these types of bugs in code by making the type requirement explicit   Type-safety is very useful  The solution to the above  7    3 would be to type cast  int   7    3  equals 10

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Type-safe means that the set of values that may be assigned to a program variable must fit well-defined and testable criteria  Type-safe variables lead to more robust programs because the algorithms that manipulate the variables can trust that the variable will only take one of a well-defined set of values  Keeping this trust ensures the integrity and quality of the data and the program    For many variables  the set of values that may be assigned to a variable is defined at the time the program is written  For example  a variable called  colour  may be allowed to take on the values  red    green   or  blue  and never any other values  For other variables those criteria may change at run-time  For example  a variable called  colour  may only be allowed to take on values in the  name  column of a  Colours  table in a relational database  where  red   green   and  blue   are three values for  name  in the  Colours  table  but some other part of the computer program may be able to add to that list while the program is running  and the variable can take on the new values after they are added to the Colours table    Many type-safe languages give the illusion of  type-safety  by insisting on strictly defining types for variables and only allowing a variable to be assigned values of the same  type   There are a couple of problems with this approach  For example  a program may have a variable  yearOfBirth  which is the year a person was born  and it is tempting to type-cast it as a short integer  However  it is not a short integer  This year  it is a number that is less than 2009 and greater than -10000  However  this set grows by 1 every year as the program runs  Making this a  short int  is not adequate  What is needed to make this variable type-safe is a run-time validation function that ensures that the number is always greater than -10000 and less than the next calendar year  There is no compiler that can enforce such criteria because these criteria are always unique characteristics of the problem domain    Languages that use dynamic typing  or duck-typing  or manifest typing  such as Perl  Python  Ruby  SQLite  and Lua don t have the notion of typed variables  This forces the programmer to write a run-time validation routine for every variable to ensure that it is correct  or endure the consequences of unexplained run-time exceptions  In my experience  programmers in statically typed languages such as C  C    Java  and C  are often lulled into thinking that statically defined types is all they need to do to get the benefits of type-safety  This is simply not true for many useful computer programs  and it is hard to predict if it is true for any particular computer program    The long  amp  the short     Do you want type-safety  If so  then write run-time functions to ensure that when a variable is assigned a value  it conforms to well-defined criteria  The down-side is that it makes domain analysis really difficult for most computer programs because you have to explicitly define the criteria for each program variable

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A programming language that is  type-safe  means following things    You can t read from uninitialized variables You can t index arrays beyond their bounds You can t perform unchecked type casts

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Type safety means that the compiler will validate types while compiling  and throw an error if you try to assign the wrong type to a variable   Some simple examples      Fails  Trying to put an integer in a string String one   1     Also fails  int foo    bar     This also applies to method arguments  since you are passing explicit types to them   int AddTwoNumbers int a  int b        return a   b      If I tried to call that using   int Sum   AddTwoNumbers 5   5      The compiler would throw an error  because I am passing a string   5    and it is expecting an integer   In a loosely typed language  such as javascript  I can do the following   function AddTwoNumbers a  b        return a   b      if I call it like this   Sum   AddTwoNumbers 5   5      Javascript automaticly converts the 5 to a string  and returns  55   This is due to javascript using the   sign for string concatenation  To make it type-aware  you would need to do something like   function AddTwoNumbers a  b        return Number a    Number b       Or  possibly   function AddOnlyTwoNumbers a  b        if  isNaN a     isNaN b           return false      return Number a    Number b       if I call it like this   Sum   AddTwoNumbers 5    dogs      Javascript automatically converts the 5 to a string  and appends them  to return  5 dogs    Not all dynamic languages are as forgiving as javascript  In fact a dynamic language does not implicity imply a loose typed language  see Python    some of them will actually give you a runtime error on invalid type casting   While its convenient  it opens you up to a lot of errors that can be easily missed  and only identified by testing the running program  Personally  I prefer to have my compiler tell me if I made that mistake   Now  back to C      C  supports a language feature called covariance  this basically means that you can substitute a base type for a child type and not cause an error  for example    public class Foo   Bar         Here  I created a new class  Foo  that subclasses Bar  I can now create a method    void DoSomething Bar myBar    And call it using either a Foo  or a Bar as an argument  both will work without causing an error  This works because C  knows that any child class of Bar will implement the interface of Bar   However  you cannot do the inverse   void DoSomething Foo myFoo    In this situation  I cannot pass Bar to this method  because the compiler does not know that Bar implements Foo s interface  This is because a child class can  and usually will  be much different than the parent class   Of course  now I ve gone way off the deep end and beyond the scope of the original question  but its all good stuff to know

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A programming language that is  type-safe  means following things    You can t read from uninitialized variables You can t index arrays beyond their bounds You can t perform unchecked type casts

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Type safety means that the compiler will validate types while compiling  and throw an error if you try to assign the wrong type to a variable   Some simple examples      Fails  Trying to put an integer in a string String one   1     Also fails  int foo    bar     This also applies to method arguments  since you are passing explicit types to them   int AddTwoNumbers int a  int b        return a   b      If I tried to call that using   int Sum   AddTwoNumbers 5   5      The compiler would throw an error  because I am passing a string   5    and it is expecting an integer   In a loosely typed language  such as javascript  I can do the following   function AddTwoNumbers a  b        return a   b      if I call it like this   Sum   AddTwoNumbers 5   5      Javascript automaticly converts the 5 to a string  and returns  55   This is due to javascript using the   sign for string concatenation  To make it type-aware  you would need to do something like   function AddTwoNumbers a  b        return Number a    Number b       Or  possibly   function AddOnlyTwoNumbers a  b        if  isNaN a     isNaN b           return false      return Number a    Number b       if I call it like this   Sum   AddTwoNumbers 5    dogs      Javascript automatically converts the 5 to a string  and appends them  to return  5 dogs    Not all dynamic languages are as forgiving as javascript  In fact a dynamic language does not implicity imply a loose typed language  see Python    some of them will actually give you a runtime error on invalid type casting   While its convenient  it opens you up to a lot of errors that can be easily missed  and only identified by testing the running program  Personally  I prefer to have my compiler tell me if I made that mistake   Now  back to C      C  supports a language feature called covariance  this basically means that you can substitute a base type for a child type and not cause an error  for example    public class Foo   Bar         Here  I created a new class  Foo  that subclasses Bar  I can now create a method    void DoSomething Bar myBar    And call it using either a Foo  or a Bar as an argument  both will work without causing an error  This works because C  knows that any child class of Bar will implement the interface of Bar   However  you cannot do the inverse   void DoSomething Foo myFoo    In this situation  I cannot pass Bar to this method  because the compiler does not know that Bar implements Foo s interface  This is because a child class can  and usually will  be much different than the parent class   Of course  now I ve gone way off the deep end and beyond the scope of the original question  but its all good stuff to know

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Try this explanation on     TypeSafe means that variables are statically checked for appropriate assignment at compile time  For example  consder a string or an integer  These two different data types cannot be cross-assigned  ie  you can t assign an integer to a string nor can you assign a string to an integer    For non-typesafe behavior  consider this   object x   89  int y    if you attempt to do this   y   x    the compiler throws an error that says it can t convert a System Object to an Integer  You need to do that explicitly  One way would be   y   Convert ToInt32  x      The assignment above is not typesafe  A typesafe assignement is where the types can directly be assigned to each other    Non typesafe collections abound in ASP NET  eg  the application  session  and viewstate collections   The good news about these collections is that  minimizing multiple server state management considerations  you can put pretty much any data type in any of the three collections  The bad news  because these collections aren t typesafe  you ll need to cast the values appropriately when you fetch them back out    For example   Session   x      34    works fine  But to assign the integer value back  you ll need to   int i   Convert ToInt32  Session   x         Read about generics for ways that facility helps you easily implement typesafe collections    C  is a typesafe language but watch for articles about C  4 0  interesting dynamic possibilities loom  is it a good thing that C  is essentially getting Option Strict  Off    we ll see

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Many answers here conflate type-safety with static-typing and dynamic-typing  A dynamically typed language  like smalltalk  can be type-safe as well   A short answer  a language is considered type-safe if no operation leads to undefined behavior  Many consider the requirement of explicit type conversions necessary for a language to be strictly typed  as automatic conversions can sometimes leads to well defined but unexpected unintuitive behaviors

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An explanation from a liberal arts major  not a comp sci major   When people say that a language or language feature is type safe  they mean that the language will help prevent you from  for example  passing something that isn t an integer to some logic that expects an integer   For example  in C   I define a function as    void foo int arg    The compiler will then stop me from doing this          call foo   foo  hello world     In other languages  the compiler would not stop me  or there is no compiler      so the string would be passed to the logic and then probably something bad will happen   Type safe languages try to catch more at  compile time    On the down side  with type safe languages  when you have a string like  123  and you want to operate on it like an int  you have to write more code to convert the string to an int  or when you have an int like 123 and want to use it in a message like   The answer is 123   you have to write more code to convert cast it to a string

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Type-safe means that the set of values that may be assigned to a program variable must fit well-defined and testable criteria  Type-safe variables lead to more robust programs because the algorithms that manipulate the variables can trust that the variable will only take one of a well-defined set of values  Keeping this trust ensures the integrity and quality of the data and the program    For many variables  the set of values that may be assigned to a variable is defined at the time the program is written  For example  a variable called  colour  may be allowed to take on the values  red    green   or  blue  and never any other values  For other variables those criteria may change at run-time  For example  a variable called  colour  may only be allowed to take on values in the  name  column of a  Colours  table in a relational database  where  red   green   and  blue   are three values for  name  in the  Colours  table  but some other part of the computer program may be able to add to that list while the program is running  and the variable can take on the new values after they are added to the Colours table    Many type-safe languages give the illusion of  type-safety  by insisting on strictly defining types for variables and only allowing a variable to be assigned values of the same  type   There are a couple of problems with this approach  For example  a program may have a variable  yearOfBirth  which is the year a person was born  and it is tempting to type-cast it as a short integer  However  it is not a short integer  This year  it is a number that is less than 2009 and greater than -10000  However  this set grows by 1 every year as the program runs  Making this a  short int  is not adequate  What is needed to make this variable type-safe is a run-time validation function that ensures that the number is always greater than -10000 and less than the next calendar year  There is no compiler that can enforce such criteria because these criteria are always unique characteristics of the problem domain    Languages that use dynamic typing  or duck-typing  or manifest typing  such as Perl  Python  Ruby  SQLite  and Lua don t have the notion of typed variables  This forces the programmer to write a run-time validation routine for every variable to ensure that it is correct  or endure the consequences of unexplained run-time exceptions  In my experience  programmers in statically typed languages such as C  C    Java  and C  are often lulled into thinking that statically defined types is all they need to do to get the benefits of type-safety  This is simply not true for many useful computer programs  and it is hard to predict if it is true for any particular computer program    The long  amp  the short     Do you want type-safety  If so  then write run-time functions to ensure that when a variable is assigned a value  it conforms to well-defined criteria  The down-side is that it makes domain analysis really difficult for most computer programs because you have to explicitly define the criteria for each program variable

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Type-safe means that programmatically  the type of data for a variable  return value  or argument must fit within a certain criteria   In practice  this means that 7  an integer type  is different from  7   a quoted character of string type    PHP  Javascript and other dynamic scripting languages are usually weakly-typed  in that they will convert a  string   7  to an  integer  7 if you try to add  7    3  although sometimes you have to do this explicitly  and Javascript uses the     character for concatenation    C C   Java will not understand that  or will concatenate the result into  73  instead  Type-safety prevents these types of bugs in code by making the type requirement explicit   Type-safety is very useful  The solution to the above  7    3 would be to type cast  int   7    3  equals 10

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Type safety means that the compiler will validate types while compiling  and throw an error if you try to assign the wrong type to a variable   Some simple examples      Fails  Trying to put an integer in a string String one   1     Also fails  int foo    bar     This also applies to method arguments  since you are passing explicit types to them   int AddTwoNumbers int a  int b        return a   b      If I tried to call that using   int Sum   AddTwoNumbers 5   5      The compiler would throw an error  because I am passing a string   5    and it is expecting an integer   In a loosely typed language  such as javascript  I can do the following   function AddTwoNumbers a  b        return a   b      if I call it like this   Sum   AddTwoNumbers 5   5      Javascript automaticly converts the 5 to a string  and returns  55   This is due to javascript using the   sign for string concatenation  To make it type-aware  you would need to do something like   function AddTwoNumbers a  b        return Number a    Number b       Or  possibly   function AddOnlyTwoNumbers a  b        if  isNaN a     isNaN b           return false      return Number a    Number b       if I call it like this   Sum   AddTwoNumbers 5    dogs      Javascript automatically converts the 5 to a string  and appends them  to return  5 dogs    Not all dynamic languages are as forgiving as javascript  In fact a dynamic language does not implicity imply a loose typed language  see Python    some of them will actually give you a runtime error on invalid type casting   While its convenient  it opens you up to a lot of errors that can be easily missed  and only identified by testing the running program  Personally  I prefer to have my compiler tell me if I made that mistake   Now  back to C      C  supports a language feature called covariance  this basically means that you can substitute a base type for a child type and not cause an error  for example    public class Foo   Bar         Here  I created a new class  Foo  that subclasses Bar  I can now create a method    void DoSomething Bar myBar    And call it using either a Foo  or a Bar as an argument  both will work without causing an error  This works because C  knows that any child class of Bar will implement the interface of Bar   However  you cannot do the inverse   void DoSomething Foo myFoo    In this situation  I cannot pass Bar to this method  because the compiler does not know that Bar implements Foo s interface  This is because a child class can  and usually will  be much different than the parent class   Of course  now I ve gone way off the deep end and beyond the scope of the original question  but its all good stuff to know

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Type-safety should not be confused with static   dynamic typing or strong   weak typing   A type-safe language is one where the only operations that one can execute on data are the ones that are condoned by the data s type  That is  if your data is of type X and X doesn t support operation y  then the language will not allow you to to execute y X    This definition doesn t set rules on when this is checked  It can be at compile time  static typing  or at runtime  dynamic typing   typically through exceptions  It can be a bit of both  some statically typed languages allow you to cast data from one type to another  and the validity of casts must be checked at runtime  imagine that you re trying to cast an Object to a Consumer - the compiler has no way of knowing whether it s acceptable or not    Type-safety does not necessarily mean strongly typed  either - some languages are notoriously weakly typed  but still arguably type safe  Take Javascript  for example  its type system is as weak as they come  but still strictly defined  It allows automatic casting of data  say  strings to ints   but within well defined rules  There is to my knowledge no case where a Javascript program will behave in an undefined fashion  and if you re clever enough  I m not   you should be able to predict what will happen when reading Javascript code   An example of a type-unsafe programming language is C  reading   writing an array value outside of the array s bounds has an undefined behaviour by specification  It s impossible to predict what will happen  C is a language that has a type system  but is not type safe

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Try this explanation on     TypeSafe means that variables are statically checked for appropriate assignment at compile time  For example  consder a string or an integer  These two different data types cannot be cross-assigned  ie  you can t assign an integer to a string nor can you assign a string to an integer    For non-typesafe behavior  consider this   object x   89  int y    if you attempt to do this   y   x    the compiler throws an error that says it can t convert a System Object to an Integer  You need to do that explicitly  One way would be   y   Convert ToInt32  x      The assignment above is not typesafe  A typesafe assignement is where the types can directly be assigned to each other    Non typesafe collections abound in ASP NET  eg  the application  session  and viewstate collections   The good news about these collections is that  minimizing multiple server state management considerations  you can put pretty much any data type in any of the three collections  The bad news  because these collections aren t typesafe  you ll need to cast the values appropriately when you fetch them back out    For example   Session   x      34    works fine  But to assign the integer value back  you ll need to   int i   Convert ToInt32  Session   x         Read about generics for ways that facility helps you easily implement typesafe collections    C  is a typesafe language but watch for articles about C  4 0  interesting dynamic possibilities loom  is it a good thing that C  is essentially getting Option Strict  Off    we ll see

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Many answers here conflate type-safety with static-typing and dynamic-typing  A dynamically typed language  like smalltalk  can be type-safe as well   A short answer  a language is considered type-safe if no operation leads to undefined behavior  Many consider the requirement of explicit type conversions necessary for a language to be strictly typed  as automatic conversions can sometimes leads to well defined but unexpected unintuitive behaviors

[language-agnostic] What is Type-safe?

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