从std :: map派生的类不能在Visual C ++上编译（但可以在gcc和clang上编译）

Question

I'm writing some container class that wraps std::map. A simplified version if it is:

#include <map>

template <typename key_type, typename value_type>
class map2 : private std::map<key_type, value_type>
{
public:
    void update(const key_type& key, value_type value)
    {
        (*this)[key] = std::move(value);
    }
};

int main()
{
    map2<int, int> m;
    m.update(1,4);
}

This code compiles just fine on gcc and clang, but on Visual C++ (I tested the 2015 version and whatever is used on http://rextester.com/l/cpp_online_compiler_visual ) it fails with:

source_file.cpp(16): error C2664: 'void map2<int,int>::update(const int &,std::pair<const _Kty,_Ty>)': cannot convert argument 2 from 'int' to 'std::pair<const _Kty,_Ty>'
        with
        [
            _Kty=int,
            _Ty=int
        ]
source_file.cpp(16): note: No constructor could take the source type, or constructor overload resolution was ambiguous
Microsoft (R) C/C++ Optimizing Compiler Version 19.00.23506 for x64

So Visual C++ somehow assumes that the value parameter of map2::update() is of type std::pair<key_type, value_type> and not of value_type . But why is it doing that, while gcc and clang accept my code just fine?

Answer 1

The problem is likely what Visual C++ is not doing, and that's two-phase name lookup. Since there's only one phase of name lookup, it will have to look up value_type in that phase. And as a hack, VC++ then already looks into dependent base classes such as std::map<key_type, value_type> here.

This incorrectly finds std::map<K, V>::value_type , a typedef for pair<K,V> . This explains the VC++ error message.

Workaround: Disambiguate the argument name in map2 yourself.