c++ - 模板不能接受对象作为参数

Question

下面的代码怎么错了？模板接受哪种类型的参数？

class MyClass
{
    int var;
};

template <MyClass a> struct s
{

};

int main()
{
    MyClass var;
    struct s<var>;


    return 0;
}

Answer 1

非类型模板参数有约束，不是什么都能用。特别是，它们必须是编译时常量，这不是你的情况。关于您的模板定义:

template <MyClass a> struct s
//        ^^^^^^^
{
};

请参阅 C++11 标准的第 14.1/4 段:

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,

— lvalue reference to object or lvalue reference to function,

— pointer to member,

— std::nullptr_t.

如您所见，不允许使用用户定义的类型。关于模板的实例化，则:

struct s<var>;
//       ^^^ You most likely meant something like s<var> obj, but nevermind

参见 C++11 标准的第 14.3.2/1 段:

A template-argument for a non-type, non-template template-parameter shall be one of:

— for a non-type template-parameter of integral or enumeration type, a converted constant expression (5.19) of the type of the template-parameter; or

— the name of a non-type template-parameter; or

— a constant expression (5.19) that designates the address of an object with static storage duration and external or internal linkage or a function with external or internal linkage, including function templates and function template-ids but excluding non-static class members, expressed (ignoring parentheses) as & id-expression, except that the & may be omitted if the name refers to a function or array and shall be omitted if the corresponding template-parameter is a reference; or

— a constant expression that evaluates to a null pointer value (4.10); or

— a constant expression that evaluates to a null member pointer value (4.11); or

— a pointer to member expressed as described in 5.3.1.