模板类的默认参数会实例化模板吗？

Question

Does the following default argument for the template instantiates a template with type EmptyClass ?

class EmptyClass{};

template <typename TYPE=EmptyClass>
class Sample
{
public:
    static void test()
    {
        TYPE::Serialize();
    }
};

Answer 1

No. Template are instantiated when used, and are instantiated on a per-function base.

Default parameter values are just the types to use when the parameter is not specified. But does not themselves imply usage.

When you call Sample<>::test() then Sample<Emptyclass>::test() is instantiated and the EmptyClass::serialize() call attempted, resulting in a compile-time error (Since Emptyclass is declared as not having such function)

Try to make up more function, containing different compile-time errors referring to different parameter, and you'll see how, until the function is not used, no error is produced.

Answer 2

No, in that code any instance of EmptyClass is created. Serialize is a static function. And EmptyClass's constructor is never called (in code showed)