Question

I used to have a normal member variable, which was initialized in the constructors as following:

ResourceSaveFunctions[OBJECTS_IDENT] = NULL;
ResourceSaveFunctions[SPRITES_IDENT] = &GMProject::SaveSprite;
ResourceSaveFunctions[SOUNDS_IDENT] = &GMProject::SaveSound;
ResourceSaveFunctions[BACKGROUNDS_IDENT] = &GMProject::SaveBackground;
ResourceSaveFunctions[PATHS_IDENT] = NULL;
ResourceSaveFunctions[SCRIPTS_IDENT] = NULL;
ResourceSaveFunctions[FONTS_IDENT] = NULL;
ResourceSaveFunctions[TIMELINES_IDENT] = NULL;
ResourceSaveFunctions[ROOMS_IDENT] = NULL;
ResourceSaveFunctions["extension"] = &GMProject::SaveExtension;
ResourceSaveFunctions[INCLUDES_IDENT] = NULL;
ResourceSaveFunctions[TRIGGERS_IDENT] = NULL;

The variable is a map with as key strings, and as data member-function-pointers. This worked perfectly fine. However as said I believe this map should be static (?) – the reason for the map is just to identify what the program should do during reading of a file. – NULL meaning “do nothing special”.

So I changed it to the following:

std::map<std::string, GMProject::GMProjectMemFn> GMProject::ResourceSaveFunctions_INIT() {
    std::map<std::string, GMProjectMemFn> tmp;
    tmp.insert(std::make_pair(OBJECTS_IDENT,NULL));
    tmp.insert(std::make_pair(SPRITES_IDENT, &GMProject::SaveSprite));
    tmp.insert(std::make_pair(SOUNDS_IDENT, &GMProject::SaveSound));
    tmp.insert(std::make_pair(BACKGROUNDS_IDENT, &GMProject::SaveBackground));
    tmp.insert(std::make_pair(PATHS_IDENT, NULL));
    tmp.insert(std::make_pair(SCRIPTS_IDENT, NULL));
    tmp.insert(std::make_pair(FONTS_IDENT, NULL));
    tmp.insert(std::make_pair(TIMELINES_IDENT, NULL));
    tmp.insert(std::make_pair(ROOMS_IDENT, NULL));
    tmp.insert(std::make_pair("extension", &GMProject::SaveExtension));
    tmp.insert(std::make_pair(INCLUDES_IDENT, NULL));
    tmp.insert(std::make_pair(TRIGGERS_IDENT, NULL));
    return tmp;
}
const std::map<std::string, GMProject::GMProjectMemFn> GMProject::ResourceSaveFunctions(GMProject::ResourceSaveFunctions_INIT());

Where those things are declared in the header:

static const std::map<std::string, GMProjectMemFn> ResourceSaveFunctions;
static std::map<std::string, GMProjectMemFn> ResourceSaveFunctions_INIT();

Now compiling suddenly brings up a lot of errors.

1>c:\program files\microsoft visual studio 10.0\vc\include\utility(163): error C2440: ‘initializing’ : cannot convert from ‘int’ to ‘GMProject::GMProjectMemFn ‘

Which is about the conversion of NULL. However shouldn’t this be just possible? Why is this not possible (yet in the previous method it was)?
Should I use an explicit cast here?

EDIT:
GMProjectMemFn defined as following:

typedef void (GMProject::*GMProjectMemFn)(const pTree&) const; 

pTree being a container.

Answer 1

std::make_pair creates a pair<T1, T2> where the types T1 and T2 are deduced implicitly from the types of the arguments. NULL expands to 0 (or 0L) so in your case make_pair returns a pair<string, int> (or a pair<string, long>).

You then try passing that pair<string, int> to map<string, GMProject::GMProjectMemFn>::insert() but this expects a pair<string, GMProjectMemFn>.

std::pair has a general copy constructor which will attempt implicit conversion of each member of the pair:

template <class U, class V>
    pair (const pair<U,V> &p) : first(p.first), second(p.second) { }

but in your case this requires converting a const int& to a pointer, which is not permitted.

In your original case you were directly converting NULL into a pointer, which is well defined.

Explicitly typing your pair should fix this:

tmp.insert(std::pair<std::string, GMProject::GMProjectMemFn>(TIMELINES_IDENT, NULL));