blocks|key|2832121|text|std::string+d+=+"hello";
const+std::string&+e+=+condition+?+d+:+std::string("world");|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|2832122|unstyled|2832123|当条件为false时，C%2B%2B是否强制延长临时对象的生命周期？|blockquote|2832124|2832125|2832126|它会是的。条件是一个右值表达式，当与const引用绑定时，编译器将创建一个未命名的对象并将引用绑定到该对象。我不能百分之百确定的是，生命周期延长的临时器是否是std::string("world")，或者(从概念上)是否制作了它的副本(并将其省略)。|offset|length|style|CODE|2832127|entityMap^0|0|0|0|0|0|I|5|27|K|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$B|C]]|$1|D|3|-4|5|E|7|T|8|@]|9|@]|A|$]]|$1|F|3|G|5|H|7|U|8|@]|9|@]|A|$]]|$1|I|3|-4|5|E|7|V|8|@]|9|@]|A|$]]|$1|J|3|-4|5|E|7|W|8|@]|9|@]|A|$]]|$1|K|3|L|5|E|7|X|8|@$M|Y|N|Z|O|P]|$M|10|N|11|O|P]]|9|@]|A|$]]|$1|Q|3|-4|5|E|7|12|8|@]|9|@]|A|$]]]|R|$]]

<pre><code>std::string d = "hello";
const std::string&amp; e = condition ? d : std::string("world");
</code></pre>

<blockquote>
 Does C++ mandate the lifetime of the temporary be extended when the condition is false?
</blockquote>

It will be. The conditional is an rvalue expression, and when bound with a <code>const</code> reference the compiler will create an unnamed object and bind the reference to it. What I am not 100% sure is whether the temporary whose lifetime is extended is <code>std::string("world")</code> or whether a copy of it is (conceptually) made (and elided).

blocks|key|3044781|text|这两个都很好。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3044782|§5.16表示(异常简略)：|3044783|如果第二个或第三个操作数的类型为空，则为|3044784|+2|blockquote|3044785|3044786|3044787|不是的。|3044788|如果第二个和第三个操作数具有不同的ﬀ类型，则返回|3044789|+3|3044790|3044791|3044792|3044793|如果第二个和第三个操作数是同一值类别的GL值，则为|3044794|+4|3044795|3044796|3044797|不是的。(在第一个中，两个都是prvalue，第二个是glvalue，一个是prvalue。)|3044798|3044799|+5否则，结果为prvalue|3044800|3044801|3044802|好的，所以这两个结果都是prvalue。所以绑定是好的，但是绑定到什么呢？|3044803|在第二个和第三个操作数上执行|3044804|+6+L值到右值(4.1)、数组到指针(4.2)和函数到指针(4.3)的标准转换。|3044805|3044806|3044807|好的，如果它们不是rvalue，那么它们现在都是rvalue。|3044808|3044809|+6(续)在这些转换之后，应满足以下条件之一:第二个和第三个操作数具有相同的类型；结果为该类型。如果操作数具有类类型，则结果是结果类型的prvalue+temporary，它是从第二个操作数或第三个操作数复制初始化的，具体取决于ﬁ第一个操作数的值。|3044810|3044811|3044812|好吧，不是std::string(first_operand)就是std::string(second_operand)。|offset|length|style|CODE|3044813|无论如何，条件表达式的结果是一个新的prvalue临时值，它是通过绑定到引用来扩展的值。|3044814|entityMap^0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|5|Q|X|R|0|0^^$0|@$1|2|3|4|5|6|7|1V|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|1W|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|1X|8|@]|9|@]|A|$]]|$1|F|3|G|5|H|7|1Y|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|1Z|8|@]|9|@]|A|$]]|$1|J|3|-4|5|6|7|20|8|@]|9|@]|A|$]]|$1|K|3|L|5|6|7|21|8|@]|9|@]|A|$]]|$1|M|3|N|5|6|7|22|8|@]|9|@]|A|$]]|$1|O|3|P|5|H|7|23|8|@]|9|@]|A|$]]|$1|Q|3|-4|5|6|7|24|8|@]|9|@]|A|$]]|$1|R|3|-4|5|6|7|25|8|@]|9|@]|A|$]]|$1|S|3|L|5|6|7|26|8|@]|9|@]|A|$]]|$1|T|3|U|5|6|7|27|8|@]|9|@]|A|$]]|$1|V|3|W|5|H|7|28|8|@]|9|@]|A|$]]|$1|X|3|-4|5|6|7|29|8|@]|9|@]|A|$]]|$1|Y|3|-4|5|6|7|2A|8|@]|9|@]|A|$]]|$1|Z|3|10|5|6|7|2B|8|@]|9|@]|A|$]]|$1|11|3|-4|5|6|7|2C|8|@]|9|@]|A|$]]|$1|12|3|13|5|H|7|2D|8|@]|9|@]|A|$]]|$1|14|3|-4|5|6|7|2E|8|@]|9|@]|A|$]]|$1|15|3|-4|5|6|7|2F|8|@]|9|@]|A|$]]|$1|16|3|17|5|6|7|2G|8|@]|9|@]|A|$]]|$1|18|3|19|5|6|7|2H|8|@]|9|@]|A|$]]|$1|1A|3|1B|5|H|7|2I|8|@]|9|@]|A|$]]|$1|1C|3|-4|5|6|7|2J|8|@]|9|@]|A|$]]|$1|1D|3|-4|5|6|7|2K|8|@]|9|@]|A|$]]|$1|1E|3|1F|5|6|7|2L|8|@]|9|@]|A|$]]|$1|1G|3|-4|5|6|7|2M|8|@]|9|@]|A|$]]|$1|1H|3|1I|5|H|7|2N|8|@]|9|@]|A|$]]|$1|1J|3|-4|5|6|7|2O|8|@]|9|@]|A|$]]|$1|1K|3|-4|5|6|7|2P|8|@]|9|@]|A|$]]|$1|1L|3|1M|5|6|7|2Q|8|@$1N|2R|1O|2S|1P|1Q]|$1N|2T|1O|2U|1P|1Q]]|9|@]|A|$]]|$1|1R|3|1S|5|6|7|2V|8|@]|9|@]|A|$]]|$1|1T|3|-4|5|6|7|2W|8|@]|9|@]|A|$]]]|1U|$]]

Both of those are fine.

§5.16 says (extraordinarily abridged):

<blockquote>
 2 If either the second or the third operand has type void
</blockquote>

Nope.

<blockquote>
 3 Otherwise, if the second and third operand have diﬀerent types 
</blockquote>

Nope.

<blockquote>
 4 If the second and third operands are glvalues of the same value category 
</blockquote>

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

<blockquote>
 5 Otherwise, the result is a prvalue
</blockquote>

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

<blockquote>
 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.
</blockquote>

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

<blockquote>
 6 (continued) After those conversions, one of the following shall hold: 
 
 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.
</blockquote>

Okay, so it's either <code>std::string(first_operand)</code> or <code>std::string(second_operand)</code>.

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.

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

<pre><code>const std::string&amp; a = std::string("hello");
std::string&amp;&amp; b = std::string("world");
</code></pre>

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

<pre><code>std::string&amp;&amp; c = condition ? std::string("hello") : std::string("world");
</code></pre>

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

<pre><code>std::string d = "hello";
const std::string&amp; e = condition ? d : std::string("world");
</code></pre>

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

The question came up while answering <a href="https://stackoverflow.com/q/14404904/252000">this question</a> about non-copyable objects.

Lifetime extension and the conditional operator

局部左值引用到常量引用和右值引用可以延长临时对象的生命周期：const std::string& a = std::string("hello");std::string&& b = std::string("world");当初始化器不是一个简单的表达式，而是使用条件运算符时，这也有效吗？std::string&& ...

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