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Type Alias & using

文章目录
  1. 1. Type alias
  2. 2. using

Type alias

Similar to typedef

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// type alias, identical to
// typedef void (*func)(int, int)

using func = void (*)(int, int);

void example(int, int) {}
func fn = example;

// alias template
template<class CharT> using mystring = std::basic_string<CharT, std::char_traits<CharT>>;
mystring<char> str;

和 <string_fwd.h> 都有以下 typedef:

typedef basic_string string;

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// type alias can introduce a member typedef name
template<typename T>
struct Container {
    using value_type = T;   // typedef T value_type;
};

template<typename Cntr>
void fn2(const Cntr& c) {
    typename Cntr::value_type n;
}

using

  1. using-directives for namespaces and using-declarations for namespace members;

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    using namespace std;
    using std::count;

  2. using-declarations for class members

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    protected:
      using _Base::_M_allocate;
      using _Base::_M_deallocate;
      using _Base::_S_nword;
      using _Base::_M_get_Bit_allocator;

  3. type alias and alias template declaration