Question

I have written a class, and I want it to work with STL algorithm sort() function.
Let me give some code snippets;

class bignum_index_cont
{
private:
mpz_class a;
long int index;
public:
bignum_index_cont() {a=0; index=0;}
bignum_index_cont(const bignum_index_cont &big) {a=big.a; index=big.index;}

void bignum_set(mpz_class &c, long int d) {a=c;index=d;}

bool operator==(bignum_index_cont &big) {return a==big.a;}
bool operator==(mpz_class &big) {return a==big;}
bool operator>(bignum_index_cont &big) { return a>big.a;}
bool operator>=(bignum_index_cont &big) {return a>=big.a;}
bool operator<(bignum_index_cont &big) {return a<big.a;}
bool operator<=(bignum_index_cont &big) {return a<=big.a;}

//some more functions..... that I think will not be needed here.
};

then I took vector<bignum_index_cont> hashtx1(pow(2,20)+1); in the global space.
now, in main() I some how managed to take inputs to all elements of vector<bignum_index_cont>
Then I decided I will sort this vector. So, I called, sort(hashtx1.begin(), hashtx1.end());..
but I got a huge list of errors. They are:

c:\mingw\bin\..\lib\gcc\mingw32\4.6.2\include\c++\bits\stl_algo.h||In function '_RandomAccessIterator std::__unguarded_partition(_RandomAccessIterator, _RandomAccessIterator, const _Tp&) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<bignum_index_cont*, std::vector<bignum_index_cont> >, _Tp = bignum_index_cont]':|
c:\mingw\bin\..\lib\gcc\mingw32\4.6.2\include\c++\bits\stl_algo.h:2253|70|instantiated from '_RandomAccessIterator std::__unguarded_partition_pivot(_RandomAccessIterator, _RandomAccessIterator) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<bignum_index_cont*, std::vector<bignum_index_cont> >]'|
c:\mingw\bin\..\lib\gcc\mingw32\4.6.2\include\c++\bits\stl_algo.h:2284|54|instantiated from 'void std::__introsort_loop(_RandomAccessIterator, _RandomAccessIterator, _Size) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<bignum_index_cont*, std::vector<bignum_index_cont> >, _Size = int]'|
c:\mingw\bin\..\lib\gcc\mingw32\4.6.2\include\c++\bits\stl_algo.h:5407|4|instantiated from 'void std::sort(_RAIter, _RAIter) [with _RAIter = __gnu_cxx::__normal_iterator<bignum_index_cont*, std::vector<bignum_index_cont> >]'|
C:\Users\poneer\Desktop\Cryptography\bignum.cpp:121|40|instantiated from here|
c:\mingw\bin\..\lib\gcc\mingw32\4.6.2\include\c++\bits\stl_algo.h|2212|error: no match for 'operator<' in '__first.__gnu_cxx::__normal_iterator<_Iterator, _Container>::operator* [with _Iterator = bignum_index_cont*, _Container = std::vector<bignum_index_cont>, __gnu_cxx::__normal_iterator<_Iterator, _Container>::reference = bignum_index_cont&]() < __pivot'|
c:\mingw\bin\..\lib\gcc\mingw32\4.6.2\include\c++\bits\stl_algo.h|2212|note: candidates are:|
C:\Users\poneer\Desktop\Cryptography\bignum.cpp|39|note: bool bignum_index_cont::operator<(bignum_index_cont&)|
C:\Users\poneer\Desktop\Cryptography\bignum.cpp|39|note:   no known conversion for argument 1 from 'const bignum_index_cont' to 'bignum_index_cont&'|
c:\mingw\bin\..\lib\gcc\mingw32\4.6.2\include\c++\bits\stl_pair.h|207|note: template<class _T1, class _T2> bool std::operator<(const std::pair<_T1, _T2>&, const std::pair<_T1, _T2>&)|
//+ a lot of errors similar to this
C:\Files\GMP\GMP\include\gmpxx.h|3150|note: template<class T, class U> bool operator<(long double, const __gmp_expr<T, U>&)|
//+ a lot similar to this

Note that, if I take vector<mpz_class> hashtx1(pow(2,20)+1) and use the sort() function, it compiles just fine.
How can I improve my code so that it works with the sort() function?

Another point is that I have even tried using a recursive quicksort() function to sort the vector. But it terminates while running.

void quicksort(long int left, long int right)
{
long int i=left, j=right;
bignum_index_cont pivot, temp;
pivot=hashtx1[(left+right)/2];

while(i<=j)
{
    while(hashtx1[i]<pivot)
        i++;
    while(hashtx1[j]>pivot)
        j--;
    if(i<=j)
    {
        temp=hashtx1[i];
        hashtx1[i]=hashtx1[j];
        hashtx1[j]=temp;
        i++; j--;
    }

}
quicksort(left, j);
quicksort(i, right);
}

I called it using quicksort(0,pow(2,20)).

Answer 1

You need to make the arguments to your comparison operators const-references and make the operators const themselves.