在析构函数中正确使用 std::uncaught_exception

有一些文章的结论是“永远不要从析构函数抛出异常”和“std::uncaught_exception() 没有用”，例如：

• http://www.gotw.ca/gotw/047.htm（赫伯·萨特撰写）

但我似乎没有明白这一点。所以我写了一个小的测试示例（见下文）。

由于测试示例一切正常，我非常感谢一些关于它可能有什么问题的评论？

测试结果：

./main

    Foo::~Foo(): caught exception - but have pending exception - ignoring
    int main(int, char**): caught exception: from int Foo::bar(int)
  

./main 1

    Foo::~Foo(): caught exception -  but *no* exception is pending - rethrowing
    int main(int, char**): caught exception: from Foo::~Foo()
  

example:

// file main.cpp
// build with e.g. "make main"
// tested successfully on Ubuntu-Karmic with g++ v4.4.1
#include <iostream>

class Foo {
  public:

  int bar(int i) {
    if (0 == i)
      throw(std::string("from ") + __PRETTY_FUNCTION__);
    else
      return i+1;
  }

  ~Foo() {
    bool exc_pending=std::uncaught_exception();
    try {
      bar(0);
    } catch (const std::string &e) {
      // ensure that no new exception has been created in the meantime
      if (std::uncaught_exception()) exc_pending = true;

      if (exc_pending) {
        std::cerr << __PRETTY_FUNCTION__ 
                  << ": caught exception - but have pending exception - ignoring"
                  << std::endl;
      } else {
        std::cerr << __PRETTY_FUNCTION__
                  << ": caught exception -  but *no* exception is pending - rethrowing"
                  << std::endl;
        throw(std::string("from ") + __PRETTY_FUNCTION__);
      }
    }
  }

};

int main(int argc, char** argv) {
  try {
    Foo f;
    // will throw an exception in Foo::bar() if no arguments given. Otherwise
    // an exception from Foo::~Foo() is thrown.
    f.bar(argc-1);
  } catch (const std::string &e) {
    std::cerr << __PRETTY_FUNCTION__ << ": caught exception: " << e << std::endl;
  }
  return 0;
}

ADDED：换句话说：尽管有些文章中有警告，但它仍按预期工作 - 那么它可能有什么问题呢？

赫伯·萨特指的是另一个问题。他正在谈论：

try
{
}
catch (...)
{
    try
    {
        // here, std::uncaught_exception() will return true
        // but it is still safe to throw an exception because
        // we have opened a new try block
    }
    catch (...)
    {
    }
}

所以问题是如果std::uncaught_exception()返回 true 你不确定是否可以安全地抛出异常。你最终必须避免抛出异常std::uncaught_exception()为了安全起见，返回 true。