Template default arguments

Question

If I am allowed to do the following:

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

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

Foo me;

But I must specify the following:

Foo<int> me;

C++11 introduced default template arguments and right now they are being elusive to my complete understanding.

Answer 1

Note:

Foo me; without template arguments is legal as of C++17. See this answer: https://stackoverflow.com/a/50970942/539997.

Original answer applicable before C++17:

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 2

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.

Answer 3

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

typedef Foo<> Fooo;

and then do

Fooo me;

Answer 4

You can use the following:

Foo<> me;

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