Difference of keywords 'typename' and 'class' in templates?

Question

For templates I have seen both declarations:

template < typename T > template < class T > 

What's the difference?

And what exactly do those keywords mean in the following example (taken from the German Wikipedia article about templates)?

template < template < typename, typename > class Container, typename Type > class Example {      Container< Type, std::allocator < Type > > baz; }; 

Answer 1

typename and class are interchangeable in the basic case of specifying a template:

template<class T> class Foo { }; 

and

template<typename T> class Foo { }; 

are equivalent.

Having said that, there are specific cases where there is a difference between typename and class.

The first one is in the case of dependent types. typename is used to declare when you are referencing a nested type that depends on another template parameter, such as the typedef in this example:

template<typename param_t> class Foo {     typedef typename param_t::baz sub_t; }; 

The second one you actually show in your question, though you might not realize it:

template < template < typename, typename > class Container, typename Type > 

When specifying a template template, the class keyword MUST be used as above -- it is not interchangeable with typename in this case (note: since C++17 both keywords are allowed in this case).

You also must use class when explicitly instantiating a template:

template class Foo<int>; 

I'm sure that there are other cases that I've missed, but the bottom line is: these two keywords are not equivalent, and these are some common cases where you need to use one or the other.