具有一个参数和默认值的类模板

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 think ¹ 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>;