如何理解"Temporary objs are destroyed as the last step in evaluating the full-expression"？谁能举个简单的例子说清楚？

Question

根据文档()，它说：

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"Temporary objects are destroyed as the last step in evaluating the full-expression ([intro.execution]) that (lexically) contains the point where they were created."？谁能举个简单的例子说清楚？

Answer 1

简单的例子。此表达式生成一个临时对象：

std::string("test")

此处，该表达式用作子表达式：

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

在 A 点，临时对象已被销毁，因为该点位于创建临时对象的完整表达式之后。

这里有一个例子，说明如果不理解这条规则，如何编写错误：

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 变得无效 - 悬空引用。将无效引用插入输出流时，行为未定义。

Answer 2

一种在很多情况下都适用的解释是，当执行到语句末尾的分号时，临时对象被销毁。某些语言结构（例如 for 循环）未包含在本说明中，因此请不要勉强解释。有关异常的更好解释，请参阅 .

举个例子：

i = foo1(foo2(std::string("test")));

临时字符串在赋值之后一直保持活动状态，因为赋值发生在语句结束之前。（在这种情况下，完整的表达式就是语句。）

如何理解"Temporary objs are destroyed as the last step in evaluating the full-expression"？谁能举个简单的例子说清楚？

How to comprehend "Temporary objs are destroyed as the last step in evaluating the full-expression"?Could anyone make it clear by some simple example?

c++

temporary-objects