std::tie 用于 C++17 结构运算符<!--?</div--><div id="text_translate"><p> 考虑以下 C++17 结构：</p><pre> struct S { M1 m1; M2 m2; M3 m3; bool operator<(const S& that) const { return tie() < that.tie(); } auto tie() const { return std::tie(m1,m2,m3); } };</pre><p> 这个对吗？ S::tie会返回成员引用的元组，还是会复制一份？ 会自动推断出正确的类型（引用元组）吗？ constness 做正确的事吗？</p><p> （我看到的示例对 std::tie 进行了两次调用，并且没有像这样分解为单独的成员 function。想知道/怀疑是否有原因。）</p></div>

Question

Consider the following C++17 struct:

struct S {
   M1 m1; M2 m2; M3 m3;

   bool operator<(const S& that) const { return tie() < that.tie(); }

   auto tie() const { return std::tie(m1,m2,m3); }
};

Is this correct? Will S::tie return a tuple of references to the members, or will it take a copy? Will auto deduce the correct type (a tuple of references)? Does the constness do the right thing?

(The examples that I've seen make two calls to std::tie and don't factor out into a seperate member function like this. Wondering / suspicious if there is a reason for that.)

Answer 1

Is this correct?

Yes, although that may depend on what you are intending to do.

Will S::tie return a tuple of references to the members

Yes.

or will it take a copy?

No. This is easy to verify by trying it with a non-copyable type.

Will auto deduce the correct type (a tuple of references)?

Yes.

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This won't be necessary nor useful anymore in C++20 with the introduction of defaulted comparison operators.