std::result_of not working with functors

std::result_of not working with functors by NikLazarov

Closed

as Fixed

Type:	Bug
ID:	732529
Opened:	3/22/2012 1:43:49 AM
Access Restriction:	Public
Moderator Decision:	Sent to Engineering Team for consideration

Workaround(s)

view

User(s) can reproduce this bug

Consider the following piece of code:

#include <functional>

struct F
{
    float operator()(int i) const { return float(i); }
};

int main()
{
    typedef ::std::result_of<F(int)>::type RetType;

    return 0;
}

It fails to compile.
This works fine with GCC 4.6. I know their implementation of result_of is different, but still... Also, GCC works both with and without "template" after "typedef"

Details (expand)

Visual Studio 2010 SP1

Try to compile:

#include <functional>

struct F
{
    float operator()(int i) const { return float(i); }
};

int main()
{
    typedef ::std::result_of<F(int)>::type RetType;

    return 0;
}

English

Windows XP

English

1>------ Rebuild All started: Project: vc2010, Configuration: Debug Win32 ------
1> main.cpp
1>c:\program files\microsoft visual studio 10.0\vc\include\xxresult(28): error C2903: 'result' : symbol is neither a class template nor a function template
1>         c:\program files\microsoft visual studio 10.0\vc\include\xxresult(40) : see reference to class template instantiation 'std::tr1::_Result_type1<__formal,_Fty,_Arg0>' being compiled
1>         with
1>         [
1>             __formal=false,
1>             _Fty=F,
1>             _Arg0=int &
1>         ]
1>         c:\program files\microsoft visual studio 10.0\vc\include\xxresult(597) : see reference to class template instantiation 'std::tr1::_Result_of1<_Fty,_Farg0>' being compiled
1>         with
1>         [
1>             _Fty=F,
1>             _Farg0=int &
1>         ]
1>         c:\program files\microsoft visual studio 10.0\vc\include\xrefwrap(28) : see reference to class template instantiation 'std::tr1::_Result_of<_Ty>' being compiled
1>         with
1>         [
1>             _Ty=F (int)
1>         ]
1>         c:\goldy\build\test\main.cpp(21) : see reference to class template instantiation 'std::tr1::result_of<_Fty>' being compiled
1>         with
1>         [
1>             _Fty=F (int)
1>         ]
1>c:\program files\microsoft visual studio 10.0\vc\include\xxresult(28): error C2039: 'result' : is not a member of 'F'
1>         c:\goldy\build\test\main.cpp(11) : see declaration of 'F'
1>c:\program files\microsoft visual studio 10.0\vc\include\xxresult(28): error C2143: syntax error : missing ';' before '<'
1>c:\program files\microsoft visual studio 10.0\vc\include\xxresult(28): error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
1>c:\program files\microsoft visual studio 10.0\vc\include\xxresult(28): error C2039: 'type' : is not a member of '`global namespace''
1>c:\program files\microsoft visual studio 10.0\vc\include\xxresult(28): error C2238: unexpected token(s) preceding ';'
1>c:\program files\microsoft visual studio 10.0\vc\include\xxresult(40): error C2039: '_Type' : is not a member of 'std::tr1::_Result_type1<__formal,_Fty,_Arg0>'
1>         with
1>         [
1>             __formal=false,
1>             _Fty=F,
1>             _Arg0=int &
1>         ]
1>c:\program files\microsoft visual studio 10.0\vc\include\xxresult(40): error C2146: syntax error : missing ';' before identifier '_Type'
1>c:\program files\microsoft visual studio 10.0\vc\include\xxresult(40): error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
1>c:\program files\microsoft visual studio 10.0\vc\include\xxresult(40): error C2602: 'std::tr1::_Result_of1<_Fty,_Farg0>::_Type' is not a member of a base class of 'std::tr1::_Result_of1<_Fty,_Farg0>'
1>         with
1>         [
1>             _Fty=F,
1>             _Farg0=int &
1>         ]
1>         c:\program files\microsoft visual studio 10.0\vc\include\xxresult(40) : see declaration of 'std::tr1::_Result_of1<_Fty,_Farg0>::_Type'
1>         with
1>         [
1>             _Fty=F,
1>             _Farg0=int &
1>         ]
1>c:\program files\microsoft visual studio 10.0\vc\include\xxresult(40): error C2868: 'std::tr1::_Result_of1<_Fty,_Farg0>::_Type' : illegal syntax for using-declaration; expected qualified-name
1>         with
1>         [
1>             _Fty=F,
1>             _Farg0=int &
1>         ]
========== Rebuild All: 0 succeeded, 1 failed, 0 skipped ==========

Should compile.

0 attachments

Hi,

Thanks for reporting this bug. We've already fixed it in VC11 Beta.

result_of was originally specified by TR1, which was a set of library extensions that could be added to a C++98/03 compiler. In C++98/03, there's no way to tell what return type you'd get if you invoked a given functor with given arguments, so result_of was specified to follow a certain protocol. Given things like function pointers, result_of could automatically determine the return type. But given functors of class type, result_of needed the return type to be manually specified, through a result_type typedef. If you add "typedef float result_type;" to F, your code will compile in VC10 SP1.

In C++11, things are different and better. The Core Language has decltype, and result_of is now powered by it. This allows it to always get the correct return type, without programmer assistance. In VC11, we've changed our result_of implementation accordingly. (This is most important for lambdas, which are compiler-generated classes that don't have result_type typedefs.)

Although decltype was originally implemented in VC10, we didn't have time back then to change result_of to use it.

If you have any further questions, feel free to E-mail me at stl@microsoft.com .

Stephan T. Lavavej
Visual C++ Libraries Developer