segfault at swap()

[nuderobmonkey]

At line 38 I get a segfault. But I think that I don't access beyond some memory section.

#include <algorithm>
#include <iostream>

template <typename Iterator, class Compare>
int part( Iterator begin, Iterator end, Compare cmp)
{    
    if( end - begin < 2 ) return end-begin;

    Iterator pivot = begin;
    Iterator left = begin + 1;
    Iterator right = end - 1;
             
    while( left < right )    
    {
        while( left < end-1 && cmp(*left,*pivot) )
        {
            ++ left;
            std::cerr << "L" <<left-pivot; // DEBUG
        }
        while( right > begin && cmp(*pivot,*right) )
        {
            -- right;
            std::cerr << "R" <<right-pivot; // DEBUG
        }
        // DEBUG
        std::cout << "|left:"<< (int)(left-begin)
         << ",right:"<<(int)(right-begin)
        << "of"<< end-begin << " ";

        if( left < right ) std::swap(*left,*right);
        std::cerr << " PART|" << begin << "| ";  // DEBUG
    }
    std::cerr << "MARK_0\n";
    // DEBUG
    std::cerr << "right:"<<(int)(right-begin)
              << " pivot:"<<(int)(pivot-begin) << '\n';
    
    std::swap( *right, *pivot );  // Here I get a segfault, why?
    std::cerr << "MARK_1\n";  //DEBUG
    return right - begin;
}

template <typename Iterator, class Compare>
void quicksort( Iterator begin, Iterator end, Compare cmp )
{
    if( end - begin < 2 ) return;
    int pivot = part( begin, end, cmp );
    quicksort( begin,  begin + pivot,  cmp );
    quicksort( begin + pivot + 1,  end,  cmp );
}

#include <string>

int main()
{
    char * test_0 = (char*)"zyx";/*wvutsrqponmlkjihgfedcba";*/
    char * test_1 = (char*)"a";
    char * test_2 = (char*)"";
    char * test = test_0;
    
    int size = 0; while( test[size] != '\0') ++size;
    std::cout << test << '\n';
    std::cout << part(test,test+size,[](char a, char b){ return a < b;} ) << '\n';
    std::cout << test << '\n';
    quicksort( test,test+size,[](char a, char b){return a < b;} );
    std::cout << test << '\n';
}

So what's wrong with?

[lastchance]

I think it is because you are attempting to swap two values, not two variables.

If I wrote swap( 2, 3 ) it wouldn't work because I had nothing to move things into.
But if I write swap( a, b ), with a and b variables it will be ok.

I'm not convinced that your partitioning routine is correct anyway.

std::swap( right, pivot );
with no deferencing will stop the segmentation fault.

[nuderobmonkey]

The error was, that the strings got protected within the execution binary.
I changed

    char * test_0 = (char*)"zyx";/*wvutsrqponmlkjihgfedcba";*/
    char * test_1 = (char*)"a";
    char * test_2 = (char*)"";

to:

    char test_0[] = "zyx";/*wvutsrqponmlkjihgfedcba";*/
    char test_1[] = "a";
    char test_2[] = "";

and now the segfault is gone.