Class template with one parameter and default value - c++

Question

I must be missing something obvious here because this really came to me as a surprise.
The following code gives me the error: error: missing template arguments before ‘a’
template<int n=0>
class A {};
...
A a;
...
Is there a reason why a template with 1 parameter declared with a default value has to be instantiated specifying a value for it?
Can someone quote the standard?

Answer 1

A is still a template class. But you can omit the template parameter though:
template<int n=0>
class A {};
int main() {
A<> a;
// ^^
return 0;
}
See the live demo.
I think1 the relevant standard section is here
14.7 Template instantiation and specialization
...
3 An explicit specialization may be declared for a function template, a class template, a member of a class
template or a member template. An explicit specialization declaration is introduced by template<>. In
an explicit specialization declaration for a class template, a member of a class template or a class member
template, the name of the class that is explicitly specialized shall be a simple-template-id. In the explicit specialization declaration for a function template or a member function template, the name of the function
or member function explicitly specialized may be a template-id.
[Example:
template<class T = int> struct A {
static int x;
};
template<class U> void g(U) { }
template<> struct A<double> { }; // specialize for T == double
template<> struct A<> { }; // specialize for T == int
template<> void g(char) { } // specialize for U == char
// U is deduced from the parameter type
template<> void g<int>(int) { } // specialize for U == int
template<> int A<char>::x = 0; // specialize for T == char
template<class T = int> struct B {
static int x;
};
template<> int B<>::x = 1; // specialize for T == int
— end example ]
1)Sorry, I can't find a better standard cite / example. It doesn't cover exactly the OP's case, but demonstrates at least the same, that templates still need the <> angle brackets to be identified as templates (classes or functions).

Answer 2

possible workaround if it's important to disguise the nature of your object:
// the template class
template<int n=0>
class ReallyA {};
// an alias for the common case
using A = ReallyA<0>;
A a;
This is how the standard library defines std::string, for example.
using string = basic_string<char>;