范围内临时对象的寿命是多少？

Question

考虑这个 class:

class Foo
{
public:

    ~ Foo ()
    {
        std::cout << "~Foo\n";
    }

    typedef std::vector<std::string> Words;

    const Words & words ()
    {
        return m_words;
    }

private:

    Words m_words = {"foo", "bar", "baz"};
};

C++ 标准的第 12.2 节指定了临时对象的生命周期。我认为这样可以：

for (auto w : Foo () .words ())
    std::cout << w << "\n";

但事实并非如此

~Foo
terminate called after throwing an instance of 'std::logic_error'
  what():  basic_string::_M_construct null not valid
[1]    10290 abort (core dumped)  ./a.out

这个标准让我很困惑。为什么在循环运行之前调用 ~Foo？

Answer 1

当前标准在The range-based for statement [stmt.ranged]中说

The range-based for statement
for ( init-statementopt for-range-declaration : for-range-initializer ) statement
is equivalent to
{
    init-statementopt
    auto &&__range = for-range-initializer ;
    auto __begin = begin-expr ;
    auto __end = end-expr ;
    for ( ; __begin != __end; ++__begin ) {
      for-range-declaration = *__begin;
      statement
    }
}

这意味着您的 Foo().words() 仅用于赋值 auto &&__range = Foo().words(); 并且临时对象在代码到达 for 循环之前不会存在。

请注意，我是从 latest C++20 draft 复制的。在 C++11 中，代码有点不同，但相关部分是相同的。

Answer 2

我认为答案可以在 range-for 的定义方式和 __range 的绑定方式中找到。

[class.temporary]/6 - There are three contexts in which temporaries are destroyed at a different point than the end of the full-expression.

The third context is when a reference is bound to a temporary object. The temporary object to which the reference is bound or the temporary object that is the complete object of a subobject to which the reference is bound persists for the lifetime of the reference ...

如果改变for-loop表达式直接绑定到子对象m_words（假设是public），那么Foo()的生命周期会延长以下将起作用

for (auto w : Foo ().m_words)
    std::cout << w << "\n";

范围内临时对象的寿命是多少？

What's the lifetime of temporary objects in a range-for?

c++

for-loop

temporary-objects

c++11