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;
}

hmm... it works? It looks like you mistyped the operator< parameter type to write frequency instead of student.

Anyway, if you want an explanation of the code that does not work: post it.

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.

Oh, I see. Frankly, I am not sure why the problem exists and how to resolve it, since I just about never define classes in functions.

Originally Posted by Mostly Harmless

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.

Why do you not want to define your class at namespace instead of function scope? You could define it in an anonymous namespace if necessary.

The problem is that locally defined types cannot be used in template arguments (and sort is a template function).

The motivation for declaring types locally is stronger with real throw-away types, like very specific predicate functors, but you cannot declare those locally either.

If you feel your Student class is a throw-away type, not useful for anything else, you could also use std::pair<int, string> (except it will sort students with the same grade in alphabetic order by name - may-be a good thing).