Template template parameters and default arguments

Question

Consider the following code which uses \"template template\" parameters to instantiate a class template using multiple types:

#include 
using         


        

Answer 1

With a using template alias, a new feature in C++11, you can create a one-parameter template that is equivalent to another template that has two parameters, one of which is defaulted.

template  using Foo1 = Foo;

This creates Foo1, a one-parameter template, even though Foo technically has two arguments, one of which is defaulted. You can use it as:

do_something(int(55));

Alternatively, if C++11 features such as using are not available, then you scan specify the default in your declaration of do_something. This means then, unfortunately, that do_something can no longer deal with simple one-arg templates. Hence, I think the using method above is better.

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