c++ initialize_list

看到这么一个东西，可以实现花括号( "{" "}" )初始化容器类。

使用时需包含头文件

#include <initialize_list>

我们都看过这样的代码

vector<int> arr = { ,,,, };

或者

vector<int> arr{ ,,,, };

右边那个花括号返回的类型便是initialize_list

我们可以在自己的类中这么用

class foo {
public:
    std::vector<int> data;
    //构造函数里放上initialize_list
    foo() {}
    foo(std::initializer_list<int> list) :data(list) {}

    void print() {
        for (auto item : data) {
            std::cout << item;
        }
        std::cout << endl;
    }
};

试验一下

int main() {
    foo test1{ ,,,, };
    foo test2 = { ,,,, };
    test1.print();
    test2.print();
    return ;
}

可以正常输出

cppreference 的测试代码如下：可以看到这个东西的花样还是挺多的

 #include <iostream>
 #include <vector>
 #include <initializer_list>

 template <class T>
 struct S {
     std::vector<T> v;
     S(std::initializer_list<T> l) : v(l) {
         std::cout << "constructed with a " << l.size() << "-element list\n";
     }
     void append(std::initializer_list<T> l) {
         v.insert(v.end(), l.begin(), l.end());
     }
     std::pair<const T*, std::size_t> c_arr() const {
         return{ &v[], v.size() };  // copy list-initialization in return statement
                                     // this is NOT a use of std::initializer_list
     }
 };

 template <typename T>
 void templated_fn(T) {}

 int main()
 {
     S<int> s = { , , , ,  }; // copy list-initialization
     s.append({ , ,  });      // list-initialization in function call

     std::cout << "The vector size is now " << s.c_arr().second << " ints:\n";

     for (auto n : s.v)
         std::cout << n << ' ';
     std::cout << '\n';

     std::cout << "Range-for over brace-init-list: \n";

     for (int x : {-, -, -}) // the rule for auto makes this ranged-for work
         std::cout << x << ' ';
     std::cout << '\n';

     auto al = { , ,  };   // special rule for auto

     std::cout << "The list bound to auto has size() = " << al.size() << '\n';

     //templated_fn({1, 2, 3}); // compiler error! "{1, 2, 3}" is not an expression,
     // it has no type, and so T cannot be deduced
     templated_fn<std::initializer_list<int>>({ , ,  }); // OK
     templated_fn<std::vector<int>>({ , ,  });           // also OK
 }