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 = { 1,2,3,4,5 };

或者

vector<int> arr{ 1,2,3,4,5 };

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{ 1,2,3,4,5 };
    foo test2 = { 1,2,3,4,5 };
    test1.print();
    test2.print();
    return 0;
}

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
";
    }
    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[0], 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 = { 1, 2, 3, 4, 5 }; // copy list-initialization
    s.append({ 6, 7, 8 });      // list-initialization in function call

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

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

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

    for (int x : {-1, -2, -3}) // the rule for auto makes this ranged-for work
        std::cout << x << ' ';
    std::cout << '
';

    auto al = { 10, 11, 12 };   // special rule for auto

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

    //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>>({ 1, 2, 3 }); // OK
    templated_fn<std::vector<int>>({ 1, 2, 3 });           // also OK
}