使用 {a,b,c} 作为参数的构造函数或 {a,b,c} 实际上在做什么？

Question

我知道，我可以像这样初始化数据。

int array[3] = { 1, 2, 3 };

甚至

int array[2][2] = { {1, 2}, {3, 4} };

我也可以用std::vector

std::vector<int> A = { 1, 2, 3 };

假设我想自己编写 class:

class my_class
{
     std::vector< int > A;
     public:
     //pseudo code
           my_class(*x) { store x in A;} //with x={ {1, 2}, {3, 4} }
     // do something
};

是否可以编写这样的构造函数，如何实现？这是什么说法

{{1, 2}, {3, 4}}实际上在做什么？

我总是发现，你可以用这种方式初始化数据，但从来不知道它到底在做什么。

Answer 1

调用list initialization and you need a std::initilizer_list构造函数，在你的my_class中实现。

参见(Live Demo)

#include <iostream>
#include <vector>
#include <initializer_list> // std::initializer_list

class my_class
{
    std::vector<int> A;
public:
    // std::initilizer_list constructor 
    my_class(const std::initializer_list<int> v) : A(v) {}    

    friend std::ostream& operator<<(std::ostream& out, const my_class& obj) /* noexcept */;
};

std::ostream& operator<<(std::ostream& out, const my_class& obj) /* noexcept */
{
    for(const int it: obj.A) out << it << " ";
    return out;
}

int main()
{
    my_class obj = {1,2,3,4};  // now possible
    std::cout << obj << std::endl;
    return 0;
}

Answer 2

您可以在构造函数中使用 initializer_list 来获得这样的选项。

struct X {
    X() = default;
    X(const X&) = default;
};

struct Q {
    Q() = default;
    Q(Q const&) = default;
    Q(std::initializer_list<Q>) {}
};

int main() {
  X x;
  X x2 = X { x }; // copy-constructor (not aggregate initialization)
  Q q;
  Q q2 = Q { q }; // initializer-list constructor (not copy constructor)
}

Answer 3

how is it possible?

如果你有初始化列表。

#include  <initializer_list> // use std::initializer_list template
my_class(const std::initializer_list<int>& v) : A(v) {}

What is this statement "{{1, 2}, {3, 4}}" actually doing?

这叫做list-initialization

If the initializer is a (non-parenthesized) braced-init-list or is = braced-init-list, the object or reference is list-initialized.

使用 {a,b,c} 作为参数的构造函数或 {a,b,c} 实际上在做什么？

Constructor using {a,b,c} as argument or what is {a,b,c} actually doing?

c++

constructor

class

c++11

stdinitializerlist