Question

template<typename T, T nontype_param>
class C {};

int a[10];
C<int *, &a[0]> err3;

The code above will throw an error when the standard of c++ is lower than 17, while it will be ok when the standard is 17. Could someone explain the reasons to me?

根据。

唯一的例外是,非类型的参考参数或点型数据和非统计数据成员,类别及其次目标非类模板参数(因为C++20)不能提及/可能是指非类型模板参数。

或以上(自C++20)项的次级目标(包括非法定类别成员、基底子或阵列元素)。

(斜体字的字体是格式的,我不确定如何在标注上正确沿用。) 如果你被我的参考点混淆,请点击该链接,并查找该部分。 iii

我认为,即使第1组,也不应把阵列的第一个要素作为非类型理由的模板。

Answer 1

<>斯特隆>;

答案取决于哪一种标准C++版本是你重新使用的。值得注意的是,a has staticstordur ;c++20允许 <><><>>>>>>>。

C++20

该方案是 https://eel.is/c++draft/temp.arg#nontype-6”rel=“nofollow noreferer”>temp.arg.nontype :

For a non-type template-parameter of reference or pointer type, or for each non-static data member of reference or pointer type in a non-type template-parameter of class type or subobject thereof, the reference or pointer value shall not refer to or be the address of (respectively):

a temporary object

a string literal object

the result of a typeid expression

a predefined func variable or

a subobject (6.7.2) of one of the above.

(斜体地雷)

Since &a[0] is neither of the above things mentioned in the list, the program is well-formed.

还指出,模板论点必须是未标明的经常表述,此处确实如此。 https://timsong-cpp.github.io/cppwp/n4861/temp.arg#nontype-2”rel=“nofollow noreferer”>temp.arg:

A template-argument for a non-type template-parameter shall be a converted constant expression ([expr.const]) of the type of the template-parameter. [ Note: If the template-argument is an overload set (or the address of such, including forming a pointer-to-member), the matching function is selected from the set ([over.over]). — end note ]

注

A converted constant expression of type T is an expression, implicitly converted to type T, where the converted expression is a constant expression and the implicit conversion sequence contains only : * ...

我们现在走到constant expression。

1. 经常表达是指通过不断表达(如下定义)而允许的某一实体,或指一种精细的核心不变表述,其价值取决于以下制约因素:

if the value is of pointer type, it contains the address of an object with static storage duration, the address past the end of such an object ([expr.add]), the address of a non-immediate function, or a null pointer value,

C++17

在C++17,该方案按temp.arg:

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,

(斜体地雷)

C++20

C++17

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