(Knowledge assumptions : Classes, Templates, Structures...)
Ever tried declaring a class, structure, array or an enum within a templated class? Perhaps, the code could be something like this:
T m_d1, m_d2, m_d3;
const MyStruct& GetStructure() const;
const MyClass<T>::MyStruct& MyClass<T>::GetStructure() const
MyStruct is not a general-purpose structure. It's used primarily for MyClass. So, it makes sense to scope it within MyClass itself. However, this code will not compile. The following errors were reported by the Microsoft Visual Studio 2005 compiler.
error: syntax error : missing ';' before '&'
error: missing type specifier - int assumed. Note: C++ does not support default-int
fatal error: unable to recover from previous error(s); stopping compilation
Clearly the cause of this lies in the fact that the compiler can't discern the type of MyStruct in the GetStructure() definition. A look at the standards provides the solution. The relevant citation is from Section 14.6:
"When a qualified-id is intended to refer to a type that is not a member of the current instantiation (18.104.22.168) and its nested-name-specifier depends on a template-parameter (14.6.2), it shall be prefixed by the keyword typename, forming a typename-specifier."
The revised GetStructure() definition for MyClass should be as follows:
const typename MyClass<T>::MyStruct& MyClass<T>::GetStructure() const