C++，没有默认构造函数的默认参数

Question

标准对这种情况怎么说:

template<class C>
struct A {
    A(C c = C());
};

struct C {
    C(int);
};

A<C> a(C(1));

顺便说一句，Comeau 不会引发错误。

Answer 1

如果您不使用默认参数(即您提供一个值)，则它根本不会被实例化。

14.7.1/2:

Unless a function template specialization has been explicitly instantiated or explicitly specialized, the func- tion template specialization is implicitly instantiated when the specialization is referenced in a context that requires a function definition to exist. Unless a call is to a function template explicit specialization or to a member function of an explicitly specialized class template, a default argument for a function template or a member function of a class template is implicitly instantiated when the function is called in a context that requires the value of the default argument.

真是一口。实际上，函数和模板的默认参数都有自己特定的实例化规则。