const 引用类成员是否会延长临时对象的生命周期? [英] Does a const reference class member prolong the life of a temporary?

问题描述

为什么会这样:

#include <string>
#include <iostream>
using namespace std;

class Sandbox
{
public:
    Sandbox(const string& n) : member(n) {}
    const string& member;
};

int main()
{
    Sandbox sandbox(string("four"));
    cout << "The answer is: " << sandbox.member << endl;
    return 0;
}

给出输出:

答案是:

代替:

答案是:四个

推荐答案

只有 local const 引用可以延长生命周期.

Only local const references prolong the lifespan.

标准在第 8.5.3/5 节 [dcl.init.ref] 中指定了此类行为，该部分关于引用声明的初始化程序.您示例中的引用绑定到构造函数的参数 n，当对象 n 绑定到超出范围时，该引用将变为无效.

The standard specifies such behavior in §8.5.3/5, [dcl.init.ref], the section on initializers of reference declarations. The reference in your example is bound to the constructor's argument n, and becomes invalid when the object n is bound to goes out of scope.

生命周期扩展不能通过函数参数传递.§12.2/5 [class.temporary]:

The lifetime extension is not transitive through a function argument. §12.2/5 [class.temporary]:

第二个上下文是引用绑定到临时对象时.引用绑定到的临时对象或作为临时对象绑定到的子对象的完整对象的临时对象在引用的生命周期内持续存在，除非在下面指定.临时绑定到构造函数的 ctor-initializer (§12.6.2 [class.base.init]) 中的引用成员会一直存在，直到构造函数退出.函数调用中引用参数的临时绑定(第 5.2.2 节 [expr.call])一直存在，直到包含调用的完整表达式完成.

The second context is when a reference is bound to a temporary. The temporary to which the reference is bound or the temporary that is the complete object to a subobject of which the temporary is bound persists for the lifetime of the reference except as specified below. A temporary bound to a reference member in a constructor’s ctor-initializer (§12.6.2 [class.base.init]) persists until the constructor exits. A temporary bound to a reference parameter in a function call (§5.2.2 [expr.call]) persists until the completion of the full expression containing the call.

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