如何使用 C++ STL Sort 对具有继承的对象数组进行排序

Question

我尝试使用 c++ STL sort() 按其中一个属性对对象数组进行排序，但我总是收到错误：

main.cpp: In function 'bool sortByArea(const Shape*, const Shape*)':
main.cpp:54:22: error: passing 'const Shape' as 'this' argument of 'double Shape::getArea()' discards qualifiers [-fpermissive]
  return lhs->getArea() < rhs->getArea();
                      ^
main.cpp:54:39: error: passing 'const Shape' as 'this' argument of 'double Shape::getArea()' discards qualifiers [-fpermissive]
  return lhs->getArea() < rhs->getArea();

这是我的代码：

形状.cpp

#include "Shape.h"

Shape::Shape(){
    width=0;
    height=0;
    area=0;
    perimeter=0;
}

Shape::Shape(double newwidth, double newheight, double newarea, double newperimeter){
    width=newwidth;
    height=newheight;
    area=newarea;
    perimeter=newperimeter;
}

Shape::~Shape(){

}

double Shape::getWidth(){
    return width;
}

double Shape::getHeight(){
    return height;
}

double Shape::getArea(){
    return area;
}

double Shape::getPerimeter(){
    return perimeter;
}

double Shape::calArea(){
    return 0;
}

double Shape::calPerimeter(){
    return 0;
}

Circle.cpp

#include "Circle.h"
#include <cmath>
#define PI 3.141592654

Circle::Circle(){
    width = height = 0;
    area=0; perimeter=0;
}

Circle::Circle(double newradius){
    width = height = newradius;
    area = calArea(); perimeter = calPerimeter();
}

Circle::~Circle(){

}

double Circle::calArea(){
    return (pow(width,2)*PI);
}

double Circle::calPerimeter(){
    return (width * PI * 2);
}

main.cpp

#include "Shape.h"
#include "Circle.h"
#include "Rectangle.h"
#include "Square.h"
#include <fstream>
#include <iostream>
#include <cstdio>
#include <algorithm>

using namespace std;
const int size = 200;
int cshape = 0; int rshape = 0; int sshape = 0;

void input_circle(Shape* mshape){
    ifstream file;
    int i;
    double r;
    file.open("circle.txt");
    while (file >> r){
        Circle crl(r);
        mshape[cshape]=crl;
        cshape++;
    }
    file.close();
}

bool sortByArea(const Shape * lhs, const Shape * rhs) { 
    return lhs->getArea() < rhs->getArea(); 
}


int main(){
    Shape* shapecir;
    shapecir = new (nothrow) Circle[size]();
    input_circle(shapecir);
    int i;
    cout << "Circle" << endl;
    sort(shapecir,shapecir+size,sortByArea);
    for (i=0;i<cshape;i++)
        cout << shapecir[i].getArea() << " " << shapecir[i].getPerimeter() << endl;
    return 0;
}

我试图在互联网上找到一些东西，但找不到任何可以提供帮助的东西。

Answer 1

您应该在编写这些函数时对其进行测试。问题出在这里：

double Shape::getArea(){
    return area;
}

bool sortByArea(const Shape * lhs, const Shape * rhs) { 
    return lhs->getArea() < rhs->getArea(); 
}

您正确地给出了 sortByArea const 指针参数，但忽略了使 getArea 成为 const 函数。编译器告诉您，在您禁止更改形状后，您正在命令代码执行可能会更改形状的操作。