Template default arguments

Question

If I am allowed to do the following:

template 
class Foo{
};

Why am I not allowed to do the following in main?

Answer 1

You are not allowed to do that but you can do this

typedef Foo<> Fooo;

and then do

Fooo me;

Answer 2

You have to do:

Foo<> me;

The template arguments must be present but you can leave them empty.

Think of it like a function foo with a single default argument. The expression foo won't call it, but foo() will. The argument syntax must still be there. This is consistent with that.

Answer 3

You can use the following:

Foo<> me;

And have int be your template argument. The angular brackets are necessary and cannot be omitted.

Answer 4

With C++17, you can indeed.

This feature is called class template argument deduction and add more flexibility to the way you can declare variables of templated types.

So,

template <typename T = int>
class Foo{};

int main() {
    Foo f;
}

is now legal C++ code.