Question

I am working on a library with two different end users one of which is using gcc 4.5.3 and the other just moved to gcc 4.6.3. The library uses the new C++11 smart pointers (in particular unique_ptr) and compiles fine on gcc 4.5.3. However, between those two versions gcc began supporting nullptr so the API of unique_ptr changed to match the standard more closely. In doing so now the following code went from fine to ambiguous

unique_ptr up( new int( 30 ) );
...
if( up == 0 ) // ambiguous call now to unique_ptr(int) for 0

Is there a clean (viz., next sentence) way to change the if statement above so that it works both with and without nullptr? I’d like to avoid a configure check and then a macro like the following (which I think will work) if possible

#if defined NULLPOINTER_AVAILABLE
  #define NULLPTR (nullptr)
#else
  #define NULLPTR (0)
#endif

or is this the only way to get the behavior I am looking?

Answer 1

What errors did you hit?

#include <iostream>
#include <memory>
int main() {
 using namespace std;
 unique_ptr<int> up( new int( 30 ) );
 if (up == 0)
     cout << "nullptr!\n";
 else cout << "bam!\n";
}

compiles fine with with g++ -std=c++0x -Wall nullptr.cpp -o nullptr (gcc 4.6.2).

Also, go through N2431 paper by Stroustrup and Sutter on nullptrwhere a similar usage (comparison with 0) is explicitly listed in one of the examples.