Class template with one parameter and default value

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?

similarly you can't call function with defaulted argument without braces. — W.F., Sep 18 '17 at 17:10
Possible duplicate of [Template default arguments](https://stackoverflow.com/questions/15373823/template-default-arguments) — Mauri, Sep 18 '17 at 17:11
@imreal `A` isn't a class, it's a template. It makes no sense to declare a variable of a template -- you want a *particular instantiation*. `<>` is the clue to the compiler that you're trying to instantiate a template, just as W.F. points out that `()` is the clue that you're trying to invoke a function (even if you aren't passing any arguments). — cdhowie, Sep 18 '17 at 17:20

user0042 · Accepted Answer · 2017-09-18T18:04:32.977

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).}

This section of the standard is more about specializing templates. I don't see anything here about instantiating templates. — cdhowie, Sep 18 '17 at 17:21
@cdhowie I gave up searching. Didn't find a better cite / example. — user0042, Sep 18 '17 at 18:05

score 3 · Answer 2 · answered Sep 18 '17 at 16:58

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>;

Class template with one parameter and default value

2 Answers2

14.7 Template instantiation and specialization