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Why I can't define compare without constant

If I define my compare function like this:

bool compare(Student& a, Student& b)
{
    return a.n < b.n;
}

g++ will complain:

g++ -Wall main.cpp -o main
In file included from /usr/lib/gcc/x86_64-pc-linux-gnu/4.5.3/include/g++-v4/algorithm:63:0,
                 from main.cpp:1:
/usr/lib/gcc/x86_64-pc-linux-gnu/4.5.3/include/g++-v4/bits/stl_algo.h: In function ‘_RandomAccessIterator std::__unguarded_partition(_RandomAccessIterator, _RandomAccessIterator, const _Tp&, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<Student*, std::vector<Student> >, _Tp = Student, _Compare = bool (*)(Student&, Student&)]’:
/usr/lib/gcc/x86_64-pc-linux-gnu/4.5.3/include/g++-v4/bits/stl_algo.h:2261:78:   instantiated from ‘_RandomAccessIterator std::__unguarded_partition_pivot(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<Student*, std::vector<Student> >, _Compare = bool (*)(Student&, Student&)]’
/usr/lib/gcc/x86_64-pc-linux-gnu/4.5.3/include/g++-v4/bits/stl_algo.h:2302:62:   instantiated from ‘void std::__introsort_loop(_RandomAccessIterator, _RandomAccessIterator, _Size, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<Student*, std::vector<Student> >, _Size = long int, _Compare = bool (*)(Student&, Student&)]’
/usr/lib/gcc/x86_64-pc-linux-gnu/4.5.3/include/g++-v4/bits/stl_algo.h:5250:4:   instantiated from ‘void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter = __gnu_cxx::__normal_iterator<Student*, std::vector<Student> >, _Compare = bool (*)(Student&, Student&)]’
main.cpp:38:51:   instantiated from here
/usr/lib/gcc/x86_64-pc-linux-gnu/4.5.3/include/g++-v4/bits/stl_algo.h:2229:4: error: invalid initialization of reference of type ‘Student&’ from expression of type ‘const Student’
/usr/lib/gcc/x86_64-pc-linux-gnu/4.5.3/include/g++-v4/bits/stl_algo.h:2232:4: error: invalid initialization of reference of type ‘Student&’ from expression of type ‘const Student’

Compilation exited abnormally with code 1 at Mon May 28 08:05:35

But if I define the compare with const type, it will compile and works fine.

And here is all the code:

class Student {
public:
    Student(string);
    string n;

};

bool compare(Student& a, Student& b)
{
    return a.n < b.n;
}

Student::Student(string name) { n = name; }

int main()
{
    Student A = Student("A");
    Student B = Student("B");

    vector<Student> students;
    students.push_back(B);
    students.push_back(A);

    sort(students.begin(), students.end(), compare);

    cout << "After sort" << endl;
    for(vector<Student>::iterator i = students.begin(); i != students.end(); ++i) {
        cout << "Student: " << i->n << endl;
    }

    return 0;
}
like image 540
Paul L Avatar asked Feb 21 '23 08:02

Paul L


1 Answers

In this implementation, std::sort uses

 const _Tp& std::__median(const _Tp&, const _Tp&, const _Tp&, _Compare);

In your case, _Tp is student, and _Compare is compare.

So you basically have

const Student& std::__median(const Student&, const Student&, const Student&, 
                                                   bool (*)(Student&, Student&) )

or similar. Obviously, the callback can't be applied to the parameters are they are converted to const, so the failure.

Make the parameters to your compare method const.

like image 118
Luchian Grigore Avatar answered Feb 25 '23 03:02

Luchian Grigore