Question

I don’t understand why this code fails to compile in main() due to “ambiguous class template instantiation” when using &T::b and &T::c. Is it a bug of g++ 4.6.1?

#include <iostream>
#include <string>
using namespace std;

struct T{
    int a;
    void b(){}
    int c()
    { 
        return 1; 
    }
};

template<typename CT, CT> struct member_helper;

template<typename FT, FT(T::*mem)> 
struct member_helper<FT(T::*), mem> {
    static string worker()
    { 
        return "for members"; 
    }
};

template<typename Return, typename... Args, Return(T::*fun)(Args...)> 
struct member_helper<Return(T::*)(Args...), fun> {
    static string worker()
    { 
        return "for member functions returning non void"; 
    }
};

template<typename... Args, void(T::*fun)(Args...)> 
struct member_helper<void(T::*)(Args...), fun> {
    static string worker()
    { 
        return "for member functions returning void"; 
    }
};

int main() {
    cout << member_helper<decltype(&T::a), &T::a>::worker();    //prints for members, ok
    cout << member_helper<decltype(&T::b), &T::b>::worker();    //cannot distinguish between all of the three
    cout << member_helper<decltype(&T::c), &T::c>::worker();    //cannot distinguish between member function returning non void and member
}

EDIT:

here’s the full error message:

g++ -O0 -g3 -Wall -c -fmessage-length=0 -std=c++0x -MMD -MP -MF"main.d" -MT"main.d" -o "main.o" "../main.cpp" 
../main.cpp: In function ‘int main()’: 
../main.cpp:27:45: error: ambiguous class template instantiation for ‘struct member_helper’ 
../main.cpp:13:43: error: candidates are: struct member_helper 
../main.cpp:17:78: error: struct member_helper 
../main.cpp:21:59: error: struct member_helper 
../main.cpp:27:8: error: incomplete type ‘member_helper’ used in nested name specifier
../main.cpp:28:45: error: ambiguous class template instantiation for ‘struct member_helper’ 
../main.cpp:13:43: error: candidates are: struct member_helper 
../main.cpp:17:78: error: struct member_helper 
../main.cpp:28:8: error: incomplete type ‘member_helper’ used in nested name specifier make: * [main.o] Errore 1

and this is the copmiler version:

Using built-in specs. COLLECT_GCC=/usr/bin/g++-4.6.real
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.6.1/lto-wrapper
Target: x86_64-linux-gnu Configured with: ../src/configure -v
–with-pkgversion=’Ubuntu/Linaro 4.6.1-9ubuntu3′ –with-bugurl=file:///usr/share/doc/gcc-4.6/README.Bugs –enable-languages=c,c++,fortran,objc,obj-c++,go –prefix=/usr –program-suffix=-4.6 –enable-shared –enable-linker-build-id –with-system-zlib –libexecdir=/usr/lib –without-included-gettext –enable-threads=posix –with-gxx-include-dir=/usr/include/c++/4.6 –libdir=/usr/lib –enable-nls –with-sysroot=/ –enable-clocale=gnu –enable-libstdcxx-debug –enable-libstdcxx-time=yes –enable-plugin –enable-objc-gc –disable-werror –with-arch-32=i686 –with-tune=generic –enable-checking=release –build=x86_64-linux-gnu –host=x86_64-linux-gnu –target=x86_64-linux-gnu Thread model: posix gcc version 4.6.1 (Ubuntu/Linaro 4.6.1-9ubuntu3)

Answer 1

This is a g++ bug — it was fixed in 4.8.x for those who run across this in the future.