Why doesn’t std::string have a virtual destructor?

问题:

When I was working on a project that involved defining sentences in a given language, I was surprised to discover that std::string destructor was not virtual. This made it a lot more difficult to specialize this class (I had to create a wrapper). Why did the standard committee decide to have this class not virtual?

in /usr/lib64/gcc/x86_64-pc-linux-gnu/4.5.3/include/g++-v4/bits/basic_string.h, we have:

template<typename _CharT, typename _Traits, typename _Alloc>
class basic_string
{
   ...

  /**
   *  @brief  Destroy the string instance.
   */
  ~basic_string()
  { _M_rep()->_M_dispose(this->get_allocator()); }

回答1:

It is by design. I think the designer is hinting that the class should not be sub-classed.

Also look at this: Why should one not derive from c++ std string class?

回答2:

It's not meant to be derived from. None of the standard classes are.

The approved way to enhance them is by encapsulation, not inheritance.