how to specify NULL as pointer-type argument to template function

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.

Your constructors don't match your object constructions. :) I figure this is for simplification, but a testcase ought to compile aside from the asked-about error. Additionally, "C2440" is a vendor-specific error code so error text would be more useful. — Lightness Races in Orbit
– Lightness Races in Orbit, Commented Jan 4, 2011 at 17:27
Why do you want to use 0 instead of NULL? This may not address the main point of your question, but isn't NULL much more readable in this case? — Jonathan Wood
– Jonathan Wood, Commented Jan 4, 2011 at 17:46

Lightness Races in Orbit · Accepted Answer · 2011-01-04 17:25:37Z

11

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

answered Jan 4, 2011 at 17:25

Lightness Races in Orbit

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1 Comment

Lightness Races in Orbit Over a year ago

(You're not wrong to think that it's weird; that is why it is getting fixed :D)

Michael Burr · Accepted Answer · 2011-01-04 17:33:27Z

5

You could use the 'make_xxx()' pattern:

#include <utility>

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

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

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

ABPair make_ABPair(A* pa, B* pb) 
{
    return ABPair(pa, pb);
}


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

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

    return 0;
}

This will let you pass not only 0, but anything that's implicitly convertible to to an A* or B*.

answered Jan 4, 2011 at 17:33

Michael Burr

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5 Comments

Lightness Races in Orbit Over a year ago

Good thinking. A little more verbose than (B*)0 but arguably neater nonetheless.

Steve Jessop Over a year ago

I'm confused, why does MSVC allow the conversion from int to B* as an argument to make_ABPair, but not as an argument to the constructor of ABPair? GCC accepts the questioner's code with -pedantic, as does Comeau. Is this a breaking change in C++0x that MSVC already supports?

Johannes Schaub - litb Over a year ago

@Steve he's using a c++0x enabled library which has a perfect forwarding constructor. So it really calls a pair(A*&&, int&&) constructor, trying to initialize the B* data member using the int parameter.

Lightness Races in Orbit Over a year ago

@Steve: It's potentially a huge breaking change in C++0x that MSVC already supports, yes.

Johannes Schaub - litb Over a year ago

@Tomalak ah NVM. It says just after the "Requires: ..." paragraph: "Remarks: If U is not implicitly convertible to first_type or V is not implicitly convertible to second_type this constructor shall not participate in overload resolution.". So this just looks like a MSVC++ lib bug.

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