Can '\0' and NULL be used interchangeably?

Question

NULL is often used in the context of pointers, and is defined via macros in multiple standard libraries (such as <iostream>) to be the integer 0. '\0' is the null character, and is 8 bits of zeros. Incidentally, 8 bits of zeros is equivalent to the integer 0.

In some cases, although it is considered to be horrible style, these two can be interchanged:

int *p='\0'; if (p==NULL) //evaluates to true     cout << "equal\n"; 

Or

char a=NULL; char b='\0'; if (a==b) //evaluates to true     cout << "equal again\n"; 

There are already many similar questions on SO alone; for example, the top answer for this question (What is the difference between NULL, '\0' and 0) says "they are not really the same thing."

Could anyone provide an example that NULL and \0 cannot be interchanged (preferably an actual application and not a pathological case)?

Answer 1

Could anyone provide an example that NULL and \0 cannot be interchanged?

The difference between NULL and '\0' may affect overload resolution.

Example (check it on Coliru):

#include <iostream>  // The overloaded function under question can be a constructor or  // an overloaded operator, which would make this example less silly void foo(char)   { std::cout << "foo(char)"  << std::endl; } void foo(int)    { std::cout << "foo(int)"   << std::endl; } void foo(long)   { std::cout << "foo(long)"  << std::endl; } void foo(void*)  { std::cout << "foo(void*)" << std::endl; }  int main() {     foo('\0'); // this will definitely call foo(char)     foo(NULL); // this, most probably, will not call foo(char) } 

Note that the gcc compiler used at Coliru defines NULL as 0L, which for this example means that foo(NULL) resolves to foo(long) rather than to foo(void*). This answer discusses that aspect in detail.