c++ - 自定义对象的常量表达式

Question

我正在尝试使用自定义类的实例作为模板参数。

class X {
public:
  X() {};
};

template <class Foo, Foo foo>
struct Bar {

};
const X x;
Bar<X, x> foo;

编译器声明 x 不能出现在常量表达式中。为什么？在编译时为构造该对象提供了一切。

Answer 1

你做不到。标准 14.1 说:

4 A non-type template-parameter shall have one of the following (optionally cv-qualified) types:
— integral or enumeration type,
— pointer to object or pointer to function,
— reference to object or reference to function,
— pointer to member.
5 [ Note: other types are disallowed either explicitly below or implicitly by the rules governing the form of template-arguments (14.3). —end note ] The top-level cv-qualifiers on the template-parameter are ignored when determining its type.