Clang 3.0 C ++ std :: map <> :: iterator编译错误

Question

I have this code:

template<typename T>
T* Factory<T>::GetObject(const char* type)
{
    StringID typeID(type);
    map<StringID, T* (*)()>::iterator it = m_createFunctions.find(typeID);
    return it->second();
}

It compiles fine on Visual Studio 2010 and 2008, but it doesn't compile on Clang 3.0 (Xcode). I think it compiled fine on GCC, but I'm not sure if it was in the same form as now. The error "; expected after expression" is on this line:

map<StringID, T* (*)()>::iterator it = m_createFunctions.find(typeID);

Do you know why?

Answer 1

VC++ accepts your code erroneously — a conformant compiler should give you an error here.

map<StringID, T* (*)()> uses T , which is a dependent type; consequently, to access types inside of map<StringID, T* (*)()> such as iterator , you need to use the typename keyword to disambiguate things for the compiler:

typename map<StringID, T* (*)()>::iterator it = m_createFunctions.find(typeID);

See this FAQ for further explanation: What is the template typename keyword used for?

---

Note that if you're compiling in C++11 mode, you can use the following simplification instead:

auto it = m_createFunctions.find(typeID);

Answer 2

You will most likely will have errors in GCC also. 'Typename' is necessary in such cases.