自定义结构和关系重载生成错误

Question

我有以下代码并收到错误消息： “错误 C2676: binary '>' : 'const Car' 没有定义此运算符或转换为预定义运算符可接受的类型”

它出现在：if (vec[i] > returnValue) /.../ 行

我知道它可能对如何比较 Car 结构感到困惑，但最后一个回调/重载函数不应该解决这个问题吗？有什么建议吗？

#include <vector>
#include <iostream>
#include <string>

struct Car {
    std::string name;
    int weight;
    int airbags;
};

//callback for typical compairsons
template <typename Type>
int CmpCallBack(Type one, Type two) {
    if (one < two) return -1;
    if (one == two) return 0;
    if (one > two) return 1;
}

//itterate through vec<Type> and return max value
template <typename Type>
Type FindMax(std::vector<Type> const &vec, int (cmpFn)(Type one, Type two) = CmpCallBack) {
    Type returnValue = new Type; //is this the right way to initialize this var?
    for (int i = 0; i < vec.size(); i++) {
        if (vec[i] > returnValue) {
            returnValue = vec[i];
        }
    }
    return returnValue;
}

//callback for the custom "Car" struct
int CarAirComp(Car one, Car two) {
    if (one.airbags < two.airbags) return -1;
    if (one.airbags == two.airbags) return 0;
    if (one.airbags > two.airbags) return 1;
}

int main () {

        //build a vector of Car types
    std::vector<Car> cars;
    Car x;
    x.airbags = 5;
    x.name = "car one";
    Car y;
    y.airbags = 3;
    y.name = "car two";
    Car z;
    z.airbags = 1;
    z.name = "car three";
    cars.push_back(x);
    cars.push_back(y);
    cars.push_back(z);

        //test function
    Car returnVal = FindMax(cars, CarAirComp);

    std::cout << "value: " << returnVal.name << std::endl;

    system("pause");
    return 0;
}

Answer 1

您没有为Car 定义operator >。编译器不知道如何评估(vec[i] > returnValue)。如果你定义了这个运算符，你应该没问题：

struct Car {
    ...
    bool operator >(const Car & other) const 
    {
       // compare them however you like
       return weight < other.weight; 
    }
}

此外，您还需要更改：

Type returnValue = new Type;

到

Type returnValue;  // this default constructs the object

更新：

因为你有一个比较功能可用，你不需要写operator >。相反，请使用您的比较功能：

if(cmpFn(vec[i], returnValue) > 0) {
    returnValue = vec[i];
}