c++中自定义的类怎么使用大括号进行赋值

Question

例如这样的一个类：

class myMatrix {
public:
    myMatrix(int row,int col);
private:
    int row;
    int col;
    double *pData;
};  

有没有办法实现如下样子的赋值？

myMatrix m(3,3) = {
    1,2,3
    4,5,6
    7,8,9
};

Answer 1

There are two methods, one is to use the constructor and/or assignment operator with parameters std::initializer_list, and the other is that this class meets the conditions of aggregate class.

Method 1

Using the constructor and/or assignment operator with parameters std::initializer_list, any class can use this method. For example:

#include <initializer_list>
#include <iostream>

// 没有考虑各种边界问题，仅为示意
class myMatrix {
public:
  myMatrix(int r, int c)
     : row(r), col(c) { pData = new double[row * col]; }
  myMatrix(int r, int c, std::initializer_list<double> il)
     : myMatrix(r, c) { *this = il; }
  ~myMatrix() { delete[] pData; }
  // Other four copy-control members are omitted here.
  myMatrix &operator=(std::initializer_list<double> il) {
    std::size_t pos = 0;
    for (const auto &e : il)
      pData[pos++] = e;
    return *this;
  }
  double get(int r, int c) const { return pData[r * col + c]; }
private:
  int row;
  int col;
  double *pData;
};

int main() {
  myMatrix m1(3, 3);
  m1 = {
    1, 2, 3,
    4, 5, 6,
    7, 8, 9
  };
  std::cout << m1.get(2, 2) << std::endl;  // 9

  myMatrix m2(4, 2, {
    11, 22,
    33, 44,
    55, 66,
    77, 88
  });
  std::cout << m2.get(2, 0) << std::endl;  // 55

  return 0;
}

Method 2

If a class meets the conditions of aggregate class, you can automatically use a brace list to initialize the members of the class.

For example, the following class satisfies the conditions of aggregate class:

struct C {
  int i;
  std::string s;
  double d;
};

So it can be initialized as follows:

C c = { 10, "abc", 6.66 };

The conditions for a class to be an aggregate class are:

• All non-static data members are public (public)

• does not define any constructor (but you can use = default or = delete)

• No base class

• No virtual member function

• No in-class initialization (deprecated after C++14)

So, the subject’s class definitely does not meet this condition.

Answer 2

用initializer_list