生命期扩展和条件运算符 [英] Lifetime extension and the conditional operator

问题描述

local lvalue references-to-const和rvalue references可以延长临时表的生命周期：

local lvalue references-to-const and rvalue references can extend the lifetime of temporaries:

const std::string& a = std::string("hello");
std::string&& b = std::string("world");

当初始化不是一个简单的表达式但是使用条件运算符时， p>

Does that also work when the initializer is not a simple expression, but uses the conditional operator?

std::string&& c = condition ? std::string("hello") : std::string("world");

如果结果之一是临时对象，而另一个结果不是？ p>

What if one of the results is a temporary object, but the other one isn't?

std::string d = "hello";
const std::string& e = condition ? d : std::string("world");

当条件为假时，C ++是否允许扩展临时的生命周期？

Does C++ mandate the lifetime of the temporary be extended when the condition is false?

问题出现在回答有关不可复制对象的此问题时。

The question came up while answering this question about non-copyable objects.

§5.16表示（非常简洁的）。

解决方案

推荐答案

）：

2如果第二个或第三个操作数具有类型void

Nope.

不适用。

3否则，如果第二个和第三个操作数有不同类型

Nope.

不适用。

4如果第二个和第三个操作数相同值类别的glvalues

Nope. (In the first, both are prvalues and in the second one is a glvalue and one is a prvalue.)

不适用。 （第一个，两个都是prvalues，第二个是glvalue，一个是prvalue。）

5 Otherwise, the result is a prvalue

5否则结果是一个prvalue

Okay, so both of these result in prvalues. So the binding is fine, but what's the binding to?

好的，所以这两个结果都是prvalues。所以绑定很好，但绑定到什么？

6 Lvalue-to-rvalue (4.1), array-to-pointer (4.2), and function-to-pointer (4.3) standard conversions are per- formed on the second and third operands.

6 Lvalue-to-rvalue（4.1），array-to-pointer

Okay, so both are now rvalues if they weren't already.

好的，第二个和第三个操作数都要执行函数到指针

6 (continued) After those conversions, one of the following shall hold:

6（续）在这些转换之后，下列其中一项应该成立： / p>

第二个和第三个操作数具有相同的类型;结果是那种类型。如果操作数具有类类型，则结果是结果类型的临时临时值，根据第一个操作数的值，从第二个操作数或第三个操作数进行复制初始化。

The second and third operands have the same type; the result is of that type. If the operands have class type, the result is a prvalue temporary of the result type, which is copy-initialized from either the second operand or the third operand depending on the value of the first operand.

好的， std :: string（first_operand）或 std :: string （second_operand）。

无论如何，条件表达式的结果是一个新的prvalue临时值，它是通过绑定到参考。

Regardless, the result of the conditional expression is a new prvalue temporary, and it's that value that's extended by binding to your references.

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