c++ - 对象作为参数传递，就像传递构造函数值一样

Question

当我在 learncpp.com 上看到以下代码时，我正在学习对象组合...所有定义都在 .h 文件中，只是为了使代码简洁。

问题是:在 main.cpp 文件中，当初始化 Creature 对象时，会传递右参数 {4,7} (我认为它调用 Point2D 的构造函数)而不是对象...它是如何工作的以及为什么？

此外，如果传递 (4,7) 而不是 {4,7}，我会收到参数不匹配的错误...为什么？

提前致谢。

Point2D.h:

#ifndef POINT2D_H
#define POINT2D_H

#include <iostream>

class Point2D
{
private:
    int m_x;
    int m_y;

public:
    // A default constructor
    Point2D()
        : m_x{ 0 }, m_y{ 0 }
    {
    }

    // A specific constructor
    Point2D(int x, int y)
        : m_x{ x }, m_y{ y }
    {
    }

    // An overloaded output operator
    friend std::ostream& operator<<(std::ostream& out, const Point2D &point)
    {
        out << '(' << point.m_x << ", " << point.m_y << ')';
        return out;
    }

    // Access functions
    void setPoint(int x, int y)
    {
        m_x = x;
        m_y = y;
    }

};

#endif

生物.h:

#ifndef CREATURE_H
#define CREATURE_H

#include <iostream>
#include <string>
#include "Point2D.h"

class Creature
{
private:
    std::string m_name;
    Point2D m_location;

public:
    Creature(const std::string &name, const Point2D &location)
        : m_name{ name }, m_location{ location }
    {
    }

    friend std::ostream& operator<<(std::ostream& out, const Creature &creature)
    {
        out << creature.m_name << " is at " << creature.m_location;
        return out;
    }

    void moveTo(int x, int y)
    {
        m_location.setPoint(x, y);
    }
};
#endif

main.cpp :

#include <string>
#include <iostream>
#include "Creature.h"
#include "Point2D.h"

int main()
{
    std::cout << "Enter a name for your creature: ";
    std::string name;
    std::cin >> name;
    Creature creature{ name, { 4, 7 }; 
// Above {4,7} is passed instead of an object

    while (true)
    {
        // print the creature's name and location
        std::cout << creature << '\n';

        std::cout << "Enter new X location for creature (-1 to quit): ";
        int x{ 0 };
        std::cin >> x;
        if (x == -1)
            break;

        std::cout << "Enter new Y location for creature (-1 to quit): ";
        int y{ 0 };
        std::cin >> y;
        if (y == -1)
            break;

        creature.moveTo(x, y);
    }

    return 0;
}```

Answer 1

当您调用此方法时:

Creature creature{ name, { 4, 7 }}; 

{4, 7} 部分被确定为 Point2D 类型，并使用复制列表初始化构造。当您使用 (4, 7) 时，这是括在括号中的逗号运算符。它的结果是值7，并且不能用于初始化Point2D。