What is the difference between "typename" and "class" template parameters?
c++, keyword, templates
Solution
`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.
Problem
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; }; ```