Question

This shows the gist of it:

#include <utility>

class A {
public:
    A() { }
};

class B {
public:
    B() { }
};

typedef std::pair<A*, B*> ABPair;

int main(int argc, char* argv[])
{
    B* b = 0;               // no C2440
    ABPair p2(new A(), b);

    ABPair p1(new A(), 0);  // C2440

    return 0;
}

Is there a better way to make the p1 declaration work than just forcing a cast, e.g.
ABPair p1(new A(), (B*)NULL)? This seems like it would be pretty common & that there would be a “right” way to do it .. and that casting it is not the right way.

On VS 2010, here’s the full error:

1>ClCompile:
1>  test.cpp
1>c:\program files (x86)\microsoft visual studio 10.0\vc\include\utility(163): error C2440: 'initializing' : cannot convert from 'int' to 'B *'
1>          Conversion from integral type to pointer type requires reinterpret_cast, C-style cast or function-style cast
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\utility(247) : see reference to function template instantiation 'std::_Pair_base<_Ty1,_Ty2>::_Pair_base<_Ty,int>(_Other1 &&,_Other2 &&)' being compiled
1>          with
1>          [
1>              _Ty1=A *,
1>              _Ty2=B *,
1>              _Ty=A *,
1>              _Other1=A *,
1>              _Other2=int
1>          ]
1>          c:\users\charliearnold\documents\visual studio 2010\projects\test\test\test.cpp(20) : see reference to function template instantiation 'std::pair<_Ty1,_Ty2>::pair<A*,int>(_Other1 &&,_Other2 &&)' being compiled
1>          with
1>          [
1>              _Ty1=A *,
1>              _Ty2=B *,
1>              _Other1=A *,
1>              _Other2=int
1>          ]
1>c:\program files (x86)\microsoft visual studio 10.0\vc\include\utility(163): error C2439: 'std::_Pair_base<_Ty1,_Ty2>::second' : member could not be initialized
1>          with
1>          [
1>              _Ty1=A *,
1>              _Ty2=B *
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\utility(167) : see declaration of 'std::_Pair_base<_Ty1,_Ty2>::second'
1>          with
1>          [
1>              _Ty1=A *,
1>              _Ty2=B *
1>          ]
1>
1>Build FAILED.

Answer 1

Not really. nullptr will fix this in C++0x, but for now, write (B*)0.