C++ SegFault with Flood Fill

Question

I want to flood fill an image, green.jpg, with red starting from (0,0). I tried doing this in C++ (because Python is slow with recursion) and used CImg as an imaging library to get and set pixels. This is my program.

#include <iostream>
#include <tuple>
#include <vector>
#include "CImg.h"

using namespace cimg_library;

int main()
{
    CImg<unsigned char> im("green.jpg");
    int l = im.width();
    int w = im.height();
    int x = 0;
    int y = 0;

    auto newcol = std::make_tuple(255,0,0);
    int R = im(x,y,0,0);
    int G = im(x,y,0,1);
    int B = im(x,y,0,2);
    auto oldcol = std::make_tuple(R,G,B);

    std::vector<std::tuple<int,int>> edge = {std::make_tuple(x,y)};

    while (edge.size() != 0)
    {
    std::vector<std::tuple<int,int>> newedge;
        for (int e = 0; e < edge.size(); e++)
        {
            int a = std::get<0>(edge[e]);
            int b = std::get<1>(edge[e]);

            if (0 <= a <= l && 0 <= b <= w) 
            {
                auto currcol = std::make_tuple(im(a,b,0,0),im(a,b,0,1),im(a,b,0,2));

                if (currcol == oldcol)
                {
                        im(a,b,0,0) = std::get<0>(newcol);
                        im(a,b,0,1) = std::get<1>(newcol);
                        im(a,b,0,2) = std::get<2>(newcol);
                        newedge.push_back(std::make_tuple(a-1,b));
                        newedge.push_back(std::make_tuple(a+1,b));
                        newedge.push_back(std::make_tuple(a,b+1));
                        newedge.push_back(std::make_tuple(a,b-1));          
                }
            }
        }
        edge = newedge;
    }

    im.save("done.jpg");    
    return 0;
}

However, I keep running into a SegFault that I can't find the cause to. I have tried using gdb to debug the program but it returns this:

Program received signal SIGSEGV, Segmentation fault.
0x000000000040b67c in std::_Head_base<2ul, unsigned char, false>::_Head_base<unsigned char&, void> (this=0x7fffffffdd10, 
__h=@0x8000f7f8c0bf: <error reading variable>) at /usr/include/c++/4.8/tuple:140

which is apparently in the tuple header file. Can someone please tell me what the SegFault is being caused by and how to fix it? Thank you.

EDIT: Added stack trace:

(gdb) bt
#0  0x000000000040b67c in std::_Head_base<2ul, unsigned char, false>::_Head_base<unsigned char&, void> (this=0x7fffffffdd10, 
    __h=@0x1006b87d1: <error reading variable>) at /usr/include/c++/4.8/tuple:140
#1  0x000000000040b6c2 in std::_Tuple_impl<2ul, unsigned char>::_Tuple_impl<unsigned char&, , void>(unsigned char&) (this=0x7fffffffdd10, 
    __head=@0x1006b87d1: <error reading variable>) at /usr/include/c++/4.8/tuple:262
#2  0x000000000040b71e in std::_Tuple_impl<1ul, unsigned char, unsigned char>::_Tuple_impl<unsigned char&, unsigned char&, void> (
    this=0x7fffffffdd10, __head=@0x1006b86f0: <error reading variable>) at /usr/include/c++/4.8/tuple:262
#3  0x000000000040b7b1 in std::_Tuple_impl<0ul, unsigned char, unsigned char, unsigned char>::_Tuple_impl<unsigned char&, unsigned char&, unsigned char&, void> (this=0x7fffffffdd10, __head=@0x1006b860f: <error reading variable>) at /usr/include/c++/4.8/tuple:262
#4  0x000000000040b832 in std::tuple<unsigned char, unsigned char, unsigned char>::tuple<unsigned char&, unsigned char&, unsigned char&, void> (
this=0x7fffffffdd10) at /usr/include/c++/4.8/tuple:405
#5  0x000000000040b895 in std::make_tuple<unsigned char&, unsigned char&, unsigned char&> () at /usr/include/c++/4.8/tuple:862
#6  0x0000000000402e5d in main () at fill.cpp:34

Answer 1

I fixed my own problem. In gdb, I found out that one of my variables, a, was equal to -1, which was outside the range of the image, causing memory problems and that segfault. if (0 <= a <= l && 0 <= b <= w) was supposed to catch this, but apparently a <= b <= c won't work properly in C++. So I changed it to if (0 <= a && a <= l && 0 <= b && b <= w) and it worked.

Thank you all for your support.