对可以传递给自动模板参数的内容有限制吗? [英] Are There Restrictions on What can be Passed to auto Template Parameters?

问题描述

在c ++ 17 我们获得了 auto模板参数.我试图用一个来传递对象，这个问题是:我可以根据算术运算写关系运算符吗?但是由 AndyG的评论指导没有编译的:(

In c++17 we got auto template parameters. I was trying to use one to pass an object in this question: Can I Write Relational Operators in Terms of Arithmetic Operations? But directed by AndyG's comment I found that didn't compile :(

使用模板功能:

template <auto T>
void foo()

我可以作为模板参数传递的内容似乎受到限制.例如，从我的链接问题中可以看出，我似乎无法传递函子:

There seem to be restrictions on what I can pass as a template parameter. For example, as seen in my linked question I cannot seem to pass functors:

foo<plus<int>{}>()

是否存在允许和不允许的清单?

Is there a list somewhere of what is and isn't allowed?

推荐答案

在C ++ 17中，可以在

In C++17, the restriction can be found in [temp.param]/4:

非类型模板参数应具有以下类型之一(可选的cv限定):

A non-type template-parameter shall have one of the following (optionally cv-qualified) types:

• 整数或枚举类型
• 指向对象的指针或指向函数的指针，
• 对对象的左值引用或对函数的左值引用，
• 成员的指针，
• std​::​nullptr_­t或
• 包含占位符类型的类型.

• 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, or
• a type that contains a placeholder type.

对 [temp.arg .nontype]/2 :

对于引用或指针类型的非类型模板参数，常量表达式的值不得引用(或对于指针类型，不得为其地址):

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):

• 一个子对象
• 一个临时对象
• 字符串文字，
• typeid表达式的结果，或者
• 预定义的_­_­func_­_变量.

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

您要出错的地方是std::plus<int>不是有效的非类型模板参数.在第一个列表中没有这些东西.

Where you're going wrong is that std::plus<int> is not a valid non-type template parameter. It is none of those things in that first list.

在C ++ 20中，可以用作非类型模板参数的类型种类将大大扩展.我们将能够使用类类型作为非类型模板参数，只要这些类类型满足强结构相等性"的要求.在当前草案中，这是根据公开定义的，默认为operator<=>.在P1185中，目前正在飞行并且可能会被采用，它将稍作更改，以公开方式定义，默认为operator==.

In C++20, the kinds of types you can use as non-type template parameters will be greatly expanded. We will be able to use class types as non-type template parameters, provided those class types satisfy something called "strong structural equality." In the current draft, that is defined in terms of public, defaulted operator<=>. In P1185, currently in flight and likely to be adopted, it will change slightly to be defined in terms of public, defaulted operator==.

但是即使在C ++ 20中，std::plus<int>实际上也没有定义任何比较运算符-因此您仍然不能将其用作非类型模板参数.

But even in C++20, std::plus<int> does not actually define any comparison operators - so you still would not be able to use it as a non-type template parameter.

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