Checking if a variable is initialized

Question

Seems like this would be a duplicate  but maybe it is just so obvious it hasn t been asked     Is this the proper way of checking if a variable  not pointer  is initialized in a C   class   class MyClass       void SomeMethod         char mCharacter      double mDecimal      void MyClass  SomeMethod         if   mCharacter                    do something with mCharacter             if     mDecimal                    define mDecimal

User · Answer

If you donot like boost and c  17  the google c   lib Abseil is another method to realize that  absl  optional is  just like std  optional    include  lt absl types optional h gt  class MyClass       void SomeMethod         absl  optional lt char gt  mCharacter      absl  optional lt double gt  mDecimal      void MyClass  SomeMethod         if  mCharacter                   do something with mCharacter             if   mDecimal                   define mDecimal

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If you mean how to check whether member variables have been initialized  you can do this by assigning them sentinel values in the constructor  Choose sentinel values as values that will never occur in normal usage of that variable  If a variables entire range is considered valid  you can create a boolean to indicate whether it has been initialized    include  lt limits gt   class MyClass       void SomeMethod         char mCharacter      bool isCharacterInitialized      double mDecimal       MyClass         isCharacterInitialized false        mDecimal  std  numeric limits lt double gt   quiet NaN                   void MyClass  SomeMethod         if   isCharacterInitialized    false                    do something with mCharacter             if   mDecimal    mDecimal      if true  mDecimal    NaN                  define mDecimal

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With C  17 you can use std  optional to check if a variable is initialized    include  lt optional gt   include  lt iostream gt      needed only for std  cout  int main         std  optional lt int gt  variable       if   variable            std  cout  lt  lt   variable is NOT initialized n              variable   3       if  variable            std  cout  lt  lt   variable IS initialized and is set to    lt  lt   variable  lt  lt    n              return 0      This will produce the output    variable is NOT initialized variable IS initialized and is set to 3     To use std  optional in the code snipped you provided  you ll have to include the  lt optional gt  standard library header and add std  optional lt     gt  to the respective variable declarations    include  lt optional gt   class MyClass       void SomeMethod         std  optional lt char gt  mCharacter      std  optional lt double gt  mDecimal      void MyClass  SomeMethod         if   mCharacter                 std  cout  lt  lt   mCharacter      do something with mCharacter             if     mDecimal                 mDecimal   3 14159      define mDecimal

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There is no way in the C   language to check whether a variable is initialized or not  although class types with constructors will be initialized automatically    Instead  what you need to do is provide constructor s  that initialize your class to a valid state  Static code checkers  and possibly some compilers  can help you find missing variables in constructors  This way you don t have to worry about being in a bogus state and the if checks in your method can go away completely

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By default  no you can t know if a variable  or pointer  has or hasn t been initialized  However  since everyone else is telling you the  easy  or  normal  approach  I ll give you something else to think about  Here s how you could keep track of something like that  no  I personally would never do this  but perhaps you have different needs than me    class MyVeryCoolInteger   public      MyVeryCoolInteger     m initialized false          MyVeryCoolInteger amp  operator  const int integer                m initialized   true          m int   integer          return  this             int value                 return m int             bool isInitialized                 return m initialized         private      int m int      bool m initialized

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Since MyClass is a POD class type  those non-static data members will have indeterminate initial values when you create a non-static instance of MyClass  so no  that is not a valid way to check if they have been initialized to a specific non-zero value     you are basically assuming they will be zero-initialized  which is not going to be the case since you have not value-initialized them in a constructor   If you want to zero-initialize your class s non-static data members  it would be best to create an initialization list and class-constructor   For example   class MyClass       void SomeMethod         char mCharacter      double mDecimal       public          MyClass        MyClass  MyClass    mCharacter 0   mDecimal 0       The initialization list in the constructor above value-initializes your data-members to zero   You can now properly assume that any non-zero value for mCharacter and mDecimal must have been specifically set by you somewhere else in your code  and contain non-zero values you can properly act on

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There s no reasonable way to check whether a value has been initialized   If you care about whether something has been initialized  instead of trying to check for it  put code into the constructor s  to ensure that they are always initialized and be done with it

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If for instance you use strings instead of chars  you might be able do something like this         a is a string of length 1     string a        b is the char in which we ll put the char stored in a     char b      bool isInitialized          if a length      NULL           b   a 0           return true         else return false

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You could reference the variable in an assertion and then build with -fsanitize address   void foo  int32 t amp  i           Assertion will trigger address sanitizer if not initialized      assert static cast lt int64 t gt  i     INT64 MAX       This will cause the program to reliably crash with a stack trace  as opposed to undefined behavior

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Depending on your applications  and especially if you re already using boost   you might want to look into boost  optional    UPDATE  As of C  17  optional is now part of the standard library  as std  optional   It has the property you are looking for  tracking whether the slot actually holds a value or not   By default it is constructed to not hold a value and evaluate to false  but if it evaluates to true you are allowed to dereference it and get the wrapped value   class MyClass       void SomeMethod         optional lt char gt  mCharacter      optional lt double gt  mDecimal      void MyClass  SomeMethod         if   mCharacter                    do something with  mCharacter              note you must use the dereference operator             if     mDecimal                    call mDecimal reset expression              this is how you assign an optional            More examples are in the Boost documentation

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There is no way of checking of the contents of a variable are undefined or not  The best thing you can do is to assign a signal sentinel value  for example in the constructor  to indicate that further initialization will need to be carried out

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With C  -11 or Boost libs you could consider storing the variable using smart pointers  Consider this MVE where toString   behaviour depends on bar being initialized or not    include  lt memory gt   include  lt sstream gt   class Foo    private      std  shared ptr lt int gt  bar   public      Foo          void setBar int bar            this- gt bar   std  make shared lt int gt  bar             std  string toString   const           std  ostringstream ss          if  bar               bar was set             ss  lt  lt   bar          else                  bar was never set             ss  lt  lt   unset           return ss str               Using this code  Foo f  std  cout  lt  lt  f toString    lt  lt  std  endl  f setBar 42   std  cout  lt  lt  f toString    lt  lt  std  endl    produces the output  unset 42

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Variable that is not defined will cause compilation error   What you re asking is about checking if it is initialized  But initialization is just a value  that you should choose and assign in the constructor   For example   class MyClass       MyClass     mCharacter  0    mDecimal -1 0         void SomeMethod         char mCharacter      double mDecimal      void MyClass  SomeMethod         if   mCharacter     0                    touched after the constructor            do something with mCharacter             if   mDecimal    -1 0                    touched after the constructor            define mDecimal            You should initialize to a default value that will mean something in the context of your logic  of course

[c++] Checking if a variable is initialized

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