c++17: 一个从未被销毁的临时对象

Question

struct Base {
    Base() {
        std::cout << "Inside:  " << __PRETTY_FUNCTION__ << std::endl;
    }
    ~Base() {
        std::cout << "Inside:  " << __PRETTY_FUNCTION__ << std::endl;
    }
};

struct BaseWrapper {
    const Base &b;
};

int main()
{
    {
        auto *w = new BaseWrapper{{}};
        std::cout << "Inside:  " << __PRETTY_FUNCTION__ << std::endl;
        delete w;
    }
    return 0;
}

当我用 C++11 或 C++14 编译上面的代码时，它按我预期的方式工作，但是当我用 C++17 编译它时，它给了我这样的东西:

Inside:  Base::Base()                                                                                                                                                              
Inside:  int main()

如您所见，Base::~Base() 从未被调用过。这对我来说没有多大意义。我已经在 Ubuntu 18.04.1 LTS 上使用 GCC 7.3.0 以及 OnlineGDB 测试了这段代码.它们都给出相同的结果。

我只是想知道这是 C++17 中的新功能还是一个错误？

更新:我很清楚 w->b 是一个悬空引用。其实，我特意写了这段代码就是为了表明这一点。然后，我在测试的时候发现了这个问题。

我真正想知道的是这个问题有多严重，以防我必须坚持使用 GCC 7.3？或者是否有任何其他方法可以重现相同的问题？还是缺陷报告？

Answer 1

很明显，创建但未销毁的临时文件是编译器错误 - 除非标准规定行为未定义。但是，该示例定义明确。标准[class.temporary]中的相关规则:

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. ...

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 exceptions to this lifetime rule are:

• ...

• A temporary bound to a reference in a new-initializer ([expr.new]) persists until the completion of the full-expression containing the new-initializer.