C++ 如何简化这段代码使得一个变量在用户输入之后才被声明

问题描述

我写了下面的代码：

#include <iostream>
using namespace std;

class Shape {
    public : float lgth;
    public : void getLgth() { cin >> lgth;}
};

class Square : public Shape {
    public : float calcArea() { return lgth * lgth; }
};

class Circle : public Shape {
    public : float calcArea() { return 3.14 * lgth * lgth; }
};

int main() {
    char shape_type;

    cout <<"Shape Type (c/s) ? ";
    cin >> shape_type;
    if (shape_type == 'c') {
      Circle my_shape;
      cout << "Shape Length ? ";
      my_shape.getLgth();
      cout << "Shape Area : " << my_shape.calcArea() << endl;
    }
    else if (shape_type == 's') {
      Square my_shape;
      cout << "Shape Length ? ";
      my_shape.getLgth();
      cout << "Shape Area : " << my_shape.calcArea() << endl;
    }

    return 0;
}

由于在执行开始时不知道形状的类型（需要用户输入），变量my_shape在一个if then else语句中被声明。在这个范围内，只有此变量被了解（我明白这一点）。因此，代码的这部分：

  cout << "Shape Length ? ";
  my_shape.getLgth();
  cout << "Shape Area : " << my_shape.calcArea() << endl;

在这个示例中，必须重复两次。（如果存在多个形状类，则需要重复更多次）

解决方案

解决方案通常是在知道要使用的类型之后动态地创建对象，并使用虚拟方法处理不同类型之间的共同交互，例如：

#include <iostream>
using namespace std;

class Shape {
    public :
        float lgth;
        void getLgth() { cin >> lgth; }
        virtual float calcArea() = 0;
};

class Square : public Shape {
    public :
        float calcArea() override { return lgth * lgth; }
};

class Circle : public Shape {
    public :
        float calcArea() override { return 3.14 * lgth * lgth; }
};

int main() {
    char shape_type;
    Shape* my_shape;

    cout << "Shape Type (c/s) ? ";
    cin >> shape_type;

    if (shape_type == 'c') {
        my_shape = new Circle;
    }
    else if (shape_type == 's') {
        my_shape = new Square;
    }
    else {
        cout << "Unknown type!" << endl;
        return -1;
    }

    cout << "Shape Length ? ";
    my_shape->getLgth();
    cout << "Shape Area : " << my_shape->calcArea() << endl;

    delete my_shape;

    return 0;
}

注意，在现代C++中，你应该使用智能指针，比如 std::unique_ptr ，而不是直接使用 new / delete ，例如：

#include <iostream>
#include <memory>
using namespace std;

class Shape {
    public :
        float lgth;
        void getLgth() { cin >> lgth; }
        virtual float calcArea() = 0;
};

class Square : public Shape {
    public :
        float calcArea() override { return lgth * lgth; }
};

class Circle : public Shape {
    public :
        float calcArea() override { return 3.14 * lgth * lgth; }
};

int main() {
    char shape_type;
    unique_ptr<Shape> my_shape;

    cout << "Shape Type (c/s) ? ";
    cin >> shape_type;

    if (shape_type == 'c') {
        my_shape = make_unique<Circle>();
    }
    else if (shape_type == 's') {
        my_shape = make_unique<Square>();
    }
    else {
        cout << "Unknown type!" << endl;
        return -1;
    }

    cout << "Shape Length ? ";
    my_shape->getLgth();
    cout << "Shape Area : " << my_shape->calcArea() << endl;

    return 0;
}