right usage of std::uncaught_exception in a destructor

Question

There are some articles concluding \"never throw an exception from a destructor\", and \"std::uncaught_exception() is not useful\", for example:

• http://www.got

Answer 1

Herb Sutter is talking about the situation when an object of class T is destroyed while there is an uncaught exception in an object of class U. std::uncaught_exception() would return true in the T destructor. The destructor would be unable to find out whether it's called during stack unwinding. If it is, it must not throw, otherwise it's business as usual.

The class U would have a problem using class T in the destructor. U would find itself dealing with a useless T object that would refuse to do anything risky in its destructor (that could include writing a log file or committing a transaction to a database).

Herb Sutter suggests never throwing in a destructor, which is a good idea. However, the C++17 offers another option. It introduced std::uncaught_exceptions(), which can be used to find out whether the destructor can throw. Following example shows the problem if complied in C++14 mode. If compiled in C++17 mode, it would work correctly.

#include 
#include 
#include 

class T
{
  public:

    ~T() noexcept(false)
    {
#if __cplusplus >= 201703L
      // C++17 - correct check
      if (std::uncaught_exceptions() == uncaught_exceptions_)
#else
      // Older C++ - incorrect check
      if (!std::uncaught_exception())
#endif
      {
        throw (std::string{__PRETTY_FUNCTION__} + " doing real work");
      }
      else
      {
        std::cerr << __PRETTY_FUNCTION__ << " cowardly quitting\n";
      }
    }

  private:

#if __cplusplus >= 201703L
    const int uncaught_exceptions_ {std::uncaught_exceptions()};
#endif
};

class U
{
  public:

    ~U()
    {
      try
      {
        T t;
      }
      catch (const std::string &e)
      {
        std::cerr << __PRETTY_FUNCTION__ << " caught: " << e << '\n';
      }
    }
};

int main()
{
  try
  {
    U u;
    throw (std::string{__PRETTY_FUNCTION__} + " threw an exception");
  }
  catch (const std::string &e)
  {
    std::cerr << __PRETTY_FUNCTION__ << " caught: " << e << '\n';
  }
  return 0;
}