C++ difference of keywords 'typename' and 'class' in templates

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

template
class Foo
{
};

and

template
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
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.

You also must use class when explicitly instantiating a template:

template class Foo;

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.