C++ 中的组合语法

Question

我正在尝试使用 C++ 中的组合和类。我遇到的代码片段以下列方式实现组合：

#include<iostream>
using namespace std;
class Engine {
  public:
    int power;
};
class Car {
  public:
    Engine e;
    string company;
    string color;
    void show_details() {
      cout << "Compnay is:            " << company << endl;
      cout << "Color is:              " << color   << endl;
      cout << "Engine horse power is: " << e.power << endl;
    }
};
int main() {
  Car c;
  c.e.power = 500;
  c.company = "hyundai";
  c.color   = "black";
  c.show_details();
  return 0;
}

这可以正常编译并运行。我不喜欢此实现的一件事是函数 "void show_details" 的实际位置，我更愿意将其放在外面。

但是，如果我天真地尝试执行以下操作：

#include<iostream>
using namespace std;
class Engine {
  public:
    int power;
};
class Car {
  public:
  //Engine e;
    string company;
    string color;
    void show_details();
  //void show_details() {
  //  cout << "Compnay is:            " << company << endl;
  //  cout << "Color is:              " << color   << endl;
  //  cout << "Engine horse power is: " << e.power << endl;
  //}
};
void Car::show_details(){
  cout << "Compnay is:            " << company << endl;
  cout << "Color is:              " << color   << endl;
  cout << "Engine horse power is: " << e.power << endl;
}
int main() {
  Car c;
  c.e.power = 500;
  c.company = "hyundai";
  c.color   = "black";
  c.show_details();
  return 0;
}

g++ returns 出现以下错误：

comp3.cpp: In member function ‘void Car::show_details()’:
comp3.cpp:22:44: error: ‘e’ was not declared in this scope
       cout << "Engine horse power is: " << e.power << endl;
                                            ^
comp3.cpp: In function ‘int main()’:
comp3.cpp:26:9: error: ‘class Car’ has no member named ‘e’
      c.e.power = 500;

我显然对范围界定问题感到困惑，但我不确定缺失的部分是什么。

感谢所有帮助！！！

Answer 1

在第二个例子中你注释掉了e。

//Engine e;

C++ 中的组合语法

Composition syntax in C++

c++

composition

scoping