三元运算的结果（类型）是什么？ [英] what is the result (type) of ternary operation?

问题描述

三元操作是否返回副本或引用？

Does the ternary operation return a copy or reference?

我检查了以下代码

vector<int> v0 = { 1, 2 };
vector<int> v1 = { 3 };

vector<int>& v = true ? v0 : v1;
v.clear(); // v0 will be cleared also

我认为三元运算会返回 v0 。然后将其传递给 v 。因此， v 和 v0 具有不同的数据存储。测试没有显示出来。

I think the ternary operation returns a copy of v0. And then pass it to v. Thus v and v0 has different storage of data. Testing doesn't show it.

谢谢，Kerrek SB！我添加了一个不应该编译的示例（感谢WhiZTiM！）来说明这一点。

Thanks, Kerrek SB! I add a "should-not-compiled" example (Thanks WhiZTiM!) to show the point.

vector<int>& v = true ? v0 : vector<int>{3};
v.clear(); // v0 will not be cleared

推荐答案

条件表达式的类型是操作数的常见类型。

The type of a conditional expression is the common type of the operands.

但是我认为您实际上对此并不感兴趣。重要的是条件表达式的值类别是什么。

But I think you aren't actually interested in that. What matters is what the value category of a conditional expression is.

如果两个操作数都是或可以转换为公共的左值类型，则条件表达式为左值；否则为右值（可能需要将其中一个操作数从左值转换为右值）。

If both operands are, or can be converted to, lvalues of the common type, then the conditional expression is an lvalue; otherwise it is an rvalue (potentially requiring lvalue-to-rvalue conversion of one of the operands).

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