Why this strange behaviour ?

[thmm]

Write your question here.
Why am I allowed to pass nullptr to a function that expects a string, but not allowed to compare the string against a nullptr ?

#include <iostream>
#include <fstream>
#include <string>
#include <cstdlib>

void f(std::string s)
{
  if (s != nullptr)
    std::cout << s.size();
}

int main()
{
  f(nullptr);

  system("pause");
  return 0; 
}

Error msg in VS 2015:
Error C2678 binary '!=': no operator found which takes a left-hand operand of type 'std::string'(or there is no acceptable conversion) in main.cpp 8

[jonnin]

I think you are seeing polymorphism. String accepts char * in its constructor and I think from assignments, and it may see null as a null char* here.

In that case the length of the string is zero.

You can't compare string and char * directly, you need to either promote to strings to use overloaded comparisons or use strcmp on the underlying char* in the string which you can get at if needed. Because of that, you can't check for null pointer on the string though --- you can check the underlying char* but I don't think that will BE null in the string.

Instead, compare the length against zero.

[jlb]

I think you are seeing polymorphism. String accepts char * in its constructor and I think from assignments, and it may see null as a null char* here.

No what you're seeing is undefined behavior. The C++ standard specifically forbids a nullptr as the first argument to the std::string constructor.

You can't compare string and char * directly

Yes you can.

The problem really has nothing to do with a char* since you're not trying to compare the std::string to a char*, you're trying to compare a std::string to a nullptr. Remember a nullptr is a type of it's own. The compiler doesn't know how to compare a nullptr to a std::string. A std::string can never be a nullptr.

By the way, after removing the if() statement the program compiles for me, but the program throws an exception when run when the compiler tries to make a copy of the std::string.

[thmm]

Thanks, I understand now.