What is the difference between ’typename’ and ’class’ in C++ template declarations?

In C++ template declarations, ’typename’ and ’class’ are used interchangeably to specify a type parameter. Both keywords allow you to define a template class or function that can work with different types.

However, there is a subtle difference between the two keywords when it comes to dependent types. A dependent type is a type that depends on a template parameter and cannot be resolved until the template is instantiated.

When using a dependent type in a template declaration, you must use the ’typename’ keyword instead of ’class’. This is because the compiler cannot determine whether a dependent name refers to a type or a non-type until it sees the template arguments. By using the ’typename’ keyword, you tell the compiler that the name refers to a type.

Here is an example that demonstrates the use of the ’typename’ keyword in a template declaration:

    template<typename T>
    class MyClass {
        public:
        typename T::iterator begin();
        typename T::iterator end();
        
        private:
        T m_data;
    };
    
    template<typename T>
    typename T::iterator MyClass<T>::begin() {
        return m_data.begin();
    }
    
    template<typename T>
    typename T::iterator MyClass<T>::end() {
        return m_data.end();
    }

In this example, the ’typename’ keyword is used to declare the return type of the ’begin()’ and ’end()’ member functions. The return type is a dependent type that depends on the type parameter ’T’, so the ’typename’ keyword is required to tell the compiler that ’T::iterator’ is a type.

Note that if you use the ’class’ keyword instead of ’typename’ in this example, you will get a compilation error.

Overall, the ’typename’ and ’class’ keywords are mostly interchangeable in C++ template declarations, but you should use ’typename’ when working with dependent types to avoid compilation errors.