error C2678: binary '=' : no operator found which takes a left-hand operand of type 'const Recipe' (or there is no acceptable conversion)

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:

In my Recipe.h file

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

In my Menu.cpp file

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

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;
};

What am I doing wrong?