error: no matching function for call to 'make_pair(int&, Quest*)'

Question

I get this weird error in g++; it compiles fine in Visual Studio.

struct Quest
{
    static map<int, Quest*> Cache;
};

Quest *Quest::LoadFromDb(BaseResult& result, int *id)
{
    Quest *ret;
    if(result.Error())
    {
        if(id)
            Cache.insert(make_pair<int, Quest*>(*id, NULL)); // <--- Problematic line

        return NULL;
    }

// ...
}

Exact error:

DataFilesStructure.cpp:9135:58: error: no matching function for call to 'make_pair(int&, Quest*)'

Answer 1

You are most probably using the C++0x version of the libstdc++ library. C++0x declares make_pair as

template <class T1, class T2>
pair<V1, V2> make_pair(T1&& x, T2&& y) noexcept;

If T1 is int, then x is int&&, and therefor cannot take lvalues of type int. Quite obviously, make_pair is designed to be called without explicit template arguments

make_pair(*id, NULL)

Answer 2

Simply remove the template parameters:

Cache.insert(make_pair(*id, NULL));

This should fix your problem.

Answer 3

Does it work with an explicit cast?

if (id)
    Cache.insert(make_pair<int, Quest*>(int(*id), NULL));

Also, a cpp file with 9000 lines, really?