Thread: Error with sorting an array of structs

The following code works when I keep the class outside of main(), but if I define the class in main(), I get the error: error: no matching function for call to ‘sort(main()::student [10], main()::student*)’ Why is this?

Code:

#include <iostream>
#include <string>
#include <algorithm>

class student
{
public:
    std::string name;
    int grade;

    bool operator<(const frequency& rhs) const
    {
        return grade < rhs.grade;
    }
};

int main()
{
    student student_array[10];

    // snip assignment of names and grades to students here

    std::sort(student_array, student_array + 10);

    return 0;
}

Oops. Should have copied and pasted rather than typed it. Here's the broken code as it stands (I made the array smaller for brevity):

Code:

#include <iostream>
#include <string>
#include <fstream>
#include <algorithm>

int main()
{
    class student
    {
    public:
        std::string name;
        int grade;
    
        bool operator<(const student& rhs) const
        {
            return grade < rhs.grade;
        }
    };
    
    student student_array[4];

    student_array[0].name = "Joe";
    student_array[0].grade = 85;
    student_array[1].name = "Sara";
    student_array[1].grade = 72;
    student_array[2].name = "Mike";
    student_array[2].grade = 98;
    student_array[3].name = "Bob";
    student_array[3].grade = 65;

    int i;

    std::cout << "UNSORTED" << std::endl;
    for(i = 0; i < 4; ++i)
    {
        std::cout << student_array[i].name << ":" << student_array[i].grade << std::endl;
    }

    std::sort(student_array, student_array + 4);

    std::cout << std::endl << "SORTED" << std::endl;
    for(i = 0; i < 4; ++i)
    {
        std::cout << student_array[i].name << ":" << student_array[i].grade << std::endl;
    }

    return 0;
}

The compiler issues:

error: no matching function for call to ‘sort(main()::student [4], main()::student*)’

If I put the class definition outside main(), if works with no problem, but I don't want to do this if I can avoid it.