在析构函数异常时销毁 return 值

Question

我有以下代码：

#include <stdexcept>
#include <iostream>

struct ok {
    int _n;
    ok(int n) : _n(n) { std::cerr << "OK" << n << " born" << std::endl; }
    ~ok() {  std::cerr << "OK" << _n << " gone" << std::endl; }
};

struct problematic {
    ~problematic() noexcept(false) { throw std::logic_error("d-tor exception"); }
};

ok boo() {
    ok ok1{1};
    problematic p;
    ok ok2{2};
    return ok{3}; // Only constructor is called...
}

int main(int argc, char **argv) {
    try {boo();} catch(...) {}
}

我看到 ok{3} 的析构函数没有被调用，输出是：

 OK1 born
 OK2 born
 OK3 born
 OK2 gone
 OK1 gone

这是 C++14 的预期行为吗？

编辑：

使用 gcc 6.3 编译

Answer 1

按照标准，这种行为是错误的，并且已经在问题的评论部分中提到了这一点。 Exception handling 部分对此进行了说明。

根据 defect reports at open-std.org，他们早在 2015 年 9 月 28 日就已经意识到实现（GCC 和 Clang）在这方面是错误的。但提议的解决方案仅在 2016 年 2 月发布，编译器（GCC 和 Clang）尚未包含此修复程序。

Proposed resolution (February, 2016):

Change 18.2 [except.ctor] paragraph 2 as follows:
The destructor is invoked for each automatic object of class type constructed, but not yet destroyed, since the try block was entered. If an exception is thrown during the destruction of temporaries or local variables for a return statement (9.6.3 [stmt.return]), the destructor for the returned object (if any) is also invoked. The objects are destroyed in the reverse order of the completion of their construction. [Example:
  struct A { };

  struct Y { ~Y() noexcept(false) { throw 0; } };

  A f() {
    try {
      A a;
      Y y;
      A b;
      return {};   // #1
    } catch (...) {
    }
    return {};     // #2
  }
At #1, the returned object of type A is constructed. Then, the local variable b is destroyed (9.6 [stmt.jump]). Next, the local variable y is destroyed, causing stack unwinding, resulting in the destruction of the returned object, followed by the destruction of the local variable a. Finally, the returned object is constructed again at #2. —end example]

在 GCC and Clang.

中已针对此问题提交了错误

GCC 错误报告的评论表明它显然是一个错误。

Jonathan Wakely 评论：

It's now 2013 so the sensible thing to do is not return by value if your destructor can throw.

另一个用户：

Yes, I noticed, and Clang has also had a bug filed against them which has languished for years. Nevertheless, the behavior is wrong.

在析构函数异常时销毁 return 值

Destruction of return value on destructor exception

c++

exception

c++14

destructor

object-lifetime