c++ - 如何理解 “Temporary objs are destroyed as the last step in evaluating the full-expression”？有人可以通过一个简单的例子弄清楚吗？

Question

根据documentation()，其中说:

When an implementation introduces a temporary object of a class that has a non-trivial constructor ([class.default.ctor], [class.copy.ctor]), it shall ensure that a constructor is called for the temporary object. Similarly, the destructor shall be called for a temporary with a non-trivial destructor ([class.dtor]). Temporary objects are destroyed as the last step in evaluating the full-expression ([intro.execution]) that (lexically) contains the point where they were created. This is true even if that evaluation ends in throwing an exception. The value computations and side effects of destroying a temporary object are associated only with the full-expression, not with any specific subexpression.

如何理解“临时对象被销毁是评估(从词法上)包含创建点的完整表达式([intro.execution])的最后一步。”今天是否可以通过一些简单的示例使它清楚？

Answer 1

简单的例子。此表达式产生一个临时对象:

std::string("test")

在这里，该表达式用作子表达式:

function(std::string("test"));
// point A

在点A，该临时对象已被破坏，因为该点位于创建临时对象的完整表达式之后。

如果无法理解此规则，这是如何编写错误的示例:

const std::string& function(const std::string& arg) {
    return arg;
}

const std::string& ref = function("test");
std::cout << ref;

在这里，作为参数创建的临时对象在完整表达式后被销毁，因此 ref变为无效-悬挂的引用。当无效的引用插入到输出流中时，行为是不确定的。