Checking for a null object in C

Question

I ve mostly only worked with C and am running into some unfamiliar issues in C     Let s say that I have some function like this in C  which would be very typical   int some c function const char  var        if  var    NULL               Exit early so we don t dereference a null pointer                 The rest of the code        And let s say that I m trying to write a similar function in C     int some cpp function const some object amp  str        if  str    NULL      This doesn t compile  probably because some object doesn t overload the    operator      if   amp str    NULL     This compiles  but it doesn t work  and does this even mean anything      Basically  all I m trying to do is to prevent the program from crashing when some cpp function   is called with NULL    What is the most typical common way of doing this with an object C    that doesn t involve overloading the    operator   Is this even the right approach  That is  should I not write functions that take an object as an argument  but rather  write member functions   but even if so  please answer the original question  Between a function that takes a reference to an object  or a function that takes a C-style pointer to an object  are there reasons to choose one over the other

User · Accepted Answer

Basically  all I m trying to do is to   prevent the program from crashing when   some cpp function   is called with   NULL    It is not possible to call the function with NULL  One of the purpose of having the reference  it will point to some object always as you have to initialize it when defining it  Do not think reference as a fancy pointer  think of it as an alias name for the object itself  Then this type of confusion will not arise

User · Answer

A reference can not be NULL  The interface makes you pass a real object into the function   So there is no need to test for NULL  This is one of the reasons that references were introduced into C     Note you can still write a function that takes a pointer  In this situation you still need to test for NULL  If the value is NULL then you return early just like in C  Note  You should not be using exceptions when a pointer is NULL  If a parameter should never be NULL then you create an interface that uses a reference

User · Answer

You can use a special designated object as the null object in case of references as follows   class SomeClass       public           int operator   SomeClass  amp object                        if this     amp object                                     return true                             return false                  static SomeClass NullObject      SomeClass SomeClass  NullObject   void print SomeClass  amp val        if val    SomeClass  NullObject                printf   nNULL              else               printf   nNOT NULL

User · Answer

What is the most typical common way of doing this with an object C    that doesn t involve overloading the    operator     Is this even the right approach  ie  should I not write functions that take an object as an argument  but rather  write member functions   But even if so  please answer the original question        No  references cannot be null  unless Undefined Behavior has already happened  in which case all bets are already off    Whether you should write a method or non-method depends on other factors         Between a function that takes a reference to an object  or a function that takes a C-style pointer to an object  are there reasons to choose one over the other       If you need to represent  no object   then pass a pointer to the function  and let that pointer be NULL   int silly sum int const  pa 0  int const  pb 0  int const  pc 0         Take up to three ints and return the sum of any supplied values     Pass null pointers for  not supplied      This is NOT an example of good code         if   pa  amp  amp   pb    pc   return silly sum pb  pc     if   pb  amp  amp  pc  return silly sum pa  pc     if  pc  return silly sum pa  pb     pc    if  pa  amp  amp  pb  return  pa    pb    if  pa  return  pa    if  pb  return  pb    return 0     int main       int a   1  b   2  c   3    cout  lt  lt  silly sum  amp a   amp b   amp c   lt  lt    n     cout  lt  lt  silly sum  amp a   amp b   lt  lt    n     cout  lt  lt  silly sum  amp a   lt  lt    n     cout  lt  lt  silly sum 0   amp b   amp c   lt  lt    n     cout  lt  lt  silly sum  amp a  0   amp c   lt  lt    n     cout  lt  lt  silly sum 0  0   amp c   lt  lt    n     return 0      If  no object  never needs to be represented  then references work fine   In fact  operator overloads are much simpler because they take overloads   You can use something like boost  optional

User · Answer

C   references naturally can t be null  you don t need the check  The function can only be called by passing a reference to an existing object

User · Answer

You should use NULL only with pointers  Your function accepts a reference and they can t be NULL   Write your function just like you would write it in C

User · Answer

As everyone said  references can t be null   That is because  a reference refers to an object   In your code      this compiles  but doesn t work  and does this even mean anything  if   amp str    NULL    you are taking the address of the object str   By definition  str exists  so it has an address   So  it cannot be NULL   So  syntactically  the above is correct  but logically  the if condition is always going to be false   About your questions  it depends upon what you want to do   Do you want the function to be able to modify the argument   If yes  pass a reference   If not  don t  or pass reference to const    See this C   FAQ for some good details   In general  in C    passing by reference is preferred by most people over passing a pointer   One of the reasons is exactly what you discovered  a reference can t be NULL  thus avoiding you the headache of checking for it in the function

User · Answer

A C   reference is not a pointer nor a Java C  style reference and cannot be NULL  They behave as if they were an alias to another existing object    In some cases  if there are bugs in your code  you might get a reference into an already dead or non-existent object  but the best thing you can do is hope that the program dies soon enough to be able to debug what happened and why your program got corrupted    That is  I have seen code checking for  null references  doing something like  if    amp reference    0    but the standard is clear that there cannot be null references in a well-formed program  If a reference is bound to a null object the program is ill-formed and should be corrected  If you need optional values  use pointers  or some higher level construct like boost  optional   not references

[c++] Checking for a null object in C++

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