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;
}
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
.
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