c++ - C++17和C++11中的非类型模板参数有什么区别？

Question

考虑这段代码:

using func = int (*)(int, int);

template<func F>
void do_something(int first, int second) {}

int something(int first, int second) { return 42; }

void  f()
{
  constexpr auto  function = something;
  do_something<function>(10, 20);
}

使用 C++17 标准兼容编译器编译和运行，但使用 C++11 标准失败:

 error: no matching function for call to ‘do_something<function>(int, int)’
   17 |   do_something<function>(10, 20);

C++11非类型模板参数和C++17非类型模板参数有什么区别？在 §14.1.4 [temp.param][n3690] 中:

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.

在 §17.1.4 [temp.param][n4713] 中:

A non-type template-parameter shall have one of the following (optionally cv-qualified) types:
(4.1) — integral or enumeration type,
(4.2) — pointer to object or pointer to function,
(4.3) — lvalue reference to object or lvalue reference to function,
(4.4) — pointer to member,
(4.5) — std::nullptr_t, or
(4.6) — a type that contains a placeholder type (10.1.7.4).

唯一的区别是:

< — a type that contains a placeholder type (10.1.7.4).

我认为这与我的问题无关，因为占位符类型类似于 auto ，并且我向模板发送了一个值，而不是占位符类型或类型。

Answer 1

相关的区别在于 [temp.arg.nontype] 中允许的模板参数(不是模板参数)的要求。
C++11:

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

• ...
• a constant expression 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
• ...

C++17 :

A template-argument for a non-type template-parameter shall be a converted constant expression of the type of the template-parameter. For a non-type template-parameter of reference or pointer type, the value of the constant expression shall not refer to (or for a pointer type, shall not be the address of):

• a subobject,
• a temporary object,
• a string literal,
• the result of a typeid expression, or
• a predefined __func__ variable.

在 C++11 中，模板参数 function不是 & 的形式id-expression，名称不引用函数 something .它引用了 int (*const)(int, int) 类型的变量，其值指向 something . (而 do_something<&function> 也无济于事，因为现在你有一个指向函数的指针，它不会转换为指向函数类型的指针。)
在 C++17 中，语法要求消失了，限制是对哪些对象不能指向或引用的更宽松的纯语义要求。