错误C2678:二进制'=':未找到采用'const Recipe'类型的左操作数的运算符(或没有可接受的转换) [英] error C2678: binary '=' : no operator found which takes a left-hand operand of type 'const Recipe' (or there is no acceptable conversion)

问题描述

我正在尝试对一个向量进行排序，该向量在每个元素中包含一个int和一个字符串.它是称为矢量配方的类类型的矢量.收到上述错误，这是我的代码:

I'm trying to sort a vector that contains an int and a string in each element. It is a vector of class type called vector recipes. Getting the above error, here's my code:

在我的Recipe.h文件中

In my Recipe.h file

struct Recipe {
public:
    string get_cname() const
    {
        return chef_name;
    }
private:
    int recipe_id;
    string chef_name;

在我的Menu.cpp文件中

In my Menu.cpp file

void Menu::show() const {
    sort(recipes.begin(), recipes.end(), Sort_by_cname());
}

在我的Menu.h文件中

In my Menu.h file

#include <vector>
#include "Recipe.h"
using namespace std;

struct Sort_by_cname 
{
    bool operator()(const Recipe& a, const Recipe& b)
    {
        return a.get_cname() < b.get_cname();
    }
};

class Menu {
public: 
    void show() const;
private
    vector<Recipe> recipes;
};

我在做什么错了?

推荐答案

Menu::show()被声明为const，因此在其内部Menu::recipes被视为已声明为std::vector<Recipe> const.

Menu::show() is declared const, so inside of it Menu::recipes is considered to have been declared as std::vector<Recipe> const.

很明显，对std::vector<>进行排序会使它变异，因此Menu::show()一定不能是const(或者Menu::recipes一定是mutable，但是在这种情况下在语义上似乎是错误的).

Obviously, sorting a std::vector<> mutates it, so Menu::show() must not be const (or Menu::recipes must be mutable, but this seems semantically incorrect in this case).

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