Visual Studio 2010 and std::function
I have this code:
#include <iostream>
#include <functional>
struct A
{
int operator()(int i) const {
std::cout << "F: " << i <<开发者_开发技巧; std::endl;
return i + 1;
}
};
int main()
{
A a;
std::tr1::function<int(int)> f = std::tr1::ref(a);
std::cout << f(6) << std::endl;
}
The aim is to pass the functor object by a reference_wrapper, in a way to avoid useless copy costructor calls. I expect the following output:
F: 6
7
It works correctly with GCC >= 4.4.0, Visual Studio 2008 and with boost by substituting std::tr1 namespace with boost. It only doesn't work with the new Visual Studio 2010 both Express Beta 2 and Release Candidate.
Are this new C++ features bugged in vs2010? Or there is some mistake or misuse in the code?
I think i found the reason. This is what TR1 3.4/2
says about result_of<T(A1, A2, ..., AN)>::type
, used in the determination of the return type of reference_wrapper<T>::operator()
:
The implementation may determine the type member via any means that produces the exact type of the expression f(t1, t2, ..., tN) for the given types. [Note: The intent is that implementations are permitted to use special compiler hooks —end note]
And then paragraph 3:
If F is not a function object defined by the standard library, and if either the implementation cannot determine the type of the expression f(t1, t2, ..., tN) or if the expression is ill-formed, the implementation shall use the following process to determine the type member:
- If F is a possibly cv-qualified class type with no member named
result_type
or iftypename F::result_type
is not a type:
- If N=0 (no arguments), type is void.
- If N>0, type is
typename F::template result<F(T1, T2,..., TN)>::type
The error message is an artefact of trying these fall-backs. Provide a typedef for result_type
to int
and it should work, i think. Notice that in C++0x
, this is different. It does not rely on result_type
or a result
template, since it can use decltype
.
If with <functional>
it fails with MSVC10 in C++0x mode, it smells like a bug, i would say. But maybe someone else knows what's going on. It may (but is not guaranteed to) work with <tr1/functional>
in C++0x mode if that header chooses to take the decltype
way instead of ::result_type
. I would typedef result_type
- that way i think it should always work regardless of whether the tr1
header is used or the c++0x
header.
Also notice that boost::tr1
says in its documentation that it does not support the function call operator (but it merely supports implicit conversions to T&
).
I encounter a similar problem here : Prevent unnecessary copies of C++ functor objects
To make it compile on MSVC10, I had to derive my function object from std::unary_function.
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