Почему я не могу определить сравнение без константы ⇐ C++

[Anonymous]

Если я определю свою функцию сравнения следующим образом:

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


g++ будет жаловаться:

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, _Tp = Student, _Compare = bool (*)(Student&, Student&)]’:
/usr/lib/gcc/x86_64-pc-linux-gnu/4.5.3/include/g++-v4/bits/stl_algo.h78: instantiated from ‘_RandomAccessIterator std::__unguarded_partition_pivot(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator, _Compare = bool (*)(Student&, Student&)]’
/usr/lib/gcc/x86_64-pc-linux-gnu/4.5.3/include/g++-v4/bits/stl_algo.h62: instantiated from ‘void std::__introsort_loop(_RandomAccessIterator, _RandomAccessIterator, _Size, _Compare) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator, _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, _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.h4: 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.h4: 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


Но если я определю тип сравнения с const, он скомпилируется и будет работать нормально.

А вот и весь код:

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 students;
students.push_back(B);
students.push_back(A);

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

cout

Подробнее здесь: https://stackoverflow.com/questions/107 ... t-constant