c++ - 自动模板参数可以传什么有限制吗？

Question

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Answer 1

我相信这完全由以下标准语句处理:

[temp.arg.nontype]
1: If the type T of a template-parameter ([temp.param]) contains a placeholder type ([dcl.spec.auto]) or a placeholder for a deduced class type ([dcl.type.class.deduct]), the type of the parameter is the type deduced for the variable x in the invented declaration

T x = template-argument ;

If a deduced parameter type is not permitted for a template-parameter declaration ([temp.param]), the program is ill-formed.

传递仿函数类型是允许的。传递仿函数实例不是，就像传递 struct A {}; 的实例一样不是。

关于允许使用哪些非类型模板参数:

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

(4.1) a type that is literal, has strong structural equality ([class.compare.default]), has no mutable or volatile subobjects, and in which if there is a defaulted member operator<=>, then it is declared public,

(4.2) an lvalue reference type,

(4.3) a type that contains a placeholder type ([dcl.spec.auto]), or

(4.4) a placeholder for a deduced class type ([dcl.type.class.deduct]).

5: [ Note: Other types are disallowed either explicitly below or implicitly by the rules governing the form of template-arguments ([temp.arg]). — end note  ] The top-level cv-qualifiers on the template-parameter are ignored when determining its type.