前向声明std :: string和std :: wstring [英] forward declaration of std::string and std::wstring

问题描述

经常讨论不能向前声明std :: string和std :: wstring的问题.据我了解，原因是这些类型是模板类basic_string的实例化的类型定义:

The problem of the inability to forward declare std::string and std::wstring is often discussed. As I understand, the reason is that those types are typedefing of instantiation of template class basic_string:

namespace std {
  typedef basic_string<char>    string;
  typedef basic_string<wchar_t> wstring;
}

该语言不允许向前声明typedef.

And forward declaration of a typedef isn't allowed by the language.

对于使用继承而不是typedef的c ++标准，会不会更好:

Wouldn't it be better for the c++ standard using inheritance instead of typedef:

namespace std {
  class string : public basic_string<char> {};
  class wstring : public basic_string<wchar_t> {};
}

这样，我们就可以向前声明std :: string和std :: wstring?

So that way we could forward declare std::string and std::wstring?

推荐答案

对于使用继承而不是typedef [...]的c ++标准来说，会更好吗?

Wouldn't be better for the c++ standard using inheritance instead of typedef [...]

否.

std :: basic_string不能以任何形式继承.此限制允许实现std :: basic_string，因为它没有虚拟函数表，因此创建和销毁它的成本更低(更快).

The std::basic_string is not meant to be inherited in any form. This limitation allows the implementation of a std::basic_string, that is much cheaper (faster) to create and destroy, as it has no virtual function table.

如果需要定义std :: [w]字符串，只需#include即可.

If you need std::[w]string to be defined, just #include it.

编辑(回答评论)

C ++(和标准库)的指导原则之一是不要为不使用的东西付费".这意味着代码是以这样一种方式编写的，即您不必为不需要的功能招致运行时费用.

One of the guiding principles of C++ (and the standard library) is "do not pay for what you do not use". This means the code is written in such a way, that you should not incur runtime costs for features you do not need.

如果每个实例都有一个虚拟表，则创建一个std :: string实例的开销将大大增加(这将使使用std :: string的做法在性能关键的代码中望而却步.)

Creating a std::string instance would be much more expensive if each instance had a virtual table (and this would make using std::string, prohibitive in performance-critical code).

相反，std :: string旨在作为C char*机制的快速，类型安全的RAII实现.同样，您不应尝试从std :: vector，列表，地图，shared_ptr，unique_ptr等继承.

Instead, std::string is intended as a fast, type-safe, RAII implementation of the C char* mechanism. Similarly, you should not attempt to inherit from std::vector, list, map, shared_ptr, unique_ptr and so on.

如果您确实需要一个字符串基类，请考虑自己编写一个your_namespace::[w]string_base(一个简单的实现就是在内部封装一个std::[w]string的实现.)

If you really need a string base class, consider writing a your_namespace::[w]string_base yourself (a trivial implementation would be one that encapsulates a std::[w]string internally).

这篇关于前向声明std :: string和std :: wstring的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！