Question

I have a static std::map<std::string, CreateGUIFunc> in a class that basically holds strings identifying gui types, and CreateGUIFunc is a reference to a factory function.

In my constructor I have

if ( m_factoryRef.size() == 0 ) {
  m_factoryRef["image"] = &CreateGUI<Image>;
  m_factoryRef["button"] = &CreateGUI<Button>;
}
...

However, this gives me an error saying assignment of read-only location ‘GUIManager::m_factoryRef.std::map<_Key, _Tp, _Compare, _Alloc>::operator[] [with _Key = std::basic_string<char, std::char_traits<char>, std::allocator<char> >, _Tp = GUI*(), _Compare = std::less<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, _Alloc = std::allocator<std::pair<const std::basic_string<char, std::char_traits<char>, std::allocator<char> >, GUI*()> >](((const std::basic_string<char, std::char_traits<char>, std::allocator<char> >&)(& std::basic_string<char, std::char_traits<char>, std::allocator<char> >(((const char*)"image"), ((const std::allocator<char>&)((const std::allocator<char>*)(& std::allocator<char>())))))))’|

I’m not sure why this is a read-only assignment. I also tried changing it to a normal member just to see if maybe it had to do with it being static, but same thing.

What is the problem?

Edit: Some definitions to make things a bit more clear

// these are all private

typedef GUI* CreateGUIFunc();

template<class T>
GUI* GUIManager::CreateGUI( std::string &filepath, int x, int y ) {
  return new T( filepath, x, y );
}

static std::map<std::string, CreateGUIFunc> m_factoryRef;

P.S. If there is any cleaner way to initialize a static map please let me know.

Answer 1

In C++, typedef GUI* CreateGUIFunc(); isn’t a function with unspecified parameters, it’s a function with NO parameters. So none of your functions match that type. What you want is typedef GUI* (*CreateGUIFunc)( std::string &filepath, int x, int y );

Next, try using the insert member function of the map instead of the subscript operator, that way you can’t end up calling the constant version by accident.