c++ - Should std::sort work with lambda function in c++0x/c++11?

Question

I tried to use lambda function with sort, but was getting "Segmentation fault" errors. I managed to simplify the code to the following:

#include <iostream>
#include <algorithm>

int main()
{
  const int len = 18;
  int intArr[len];
  for (int i=0;i<len;i++) intArr[i]=1000+i;
  // The following is expected to sort all but the last element of the array
  std::sort(intArr, intArr + len -1, [](int a, int b)
    {
      std::cout<<"("<<a<<", "<<b<<")
";
      return (a<b?-1:(a>b?1:0));
    });
  return 0;
}

I compile and run this code in Ubuntu 11.04 (x64) using

g++ -std=gnu++0x test2.cpp && ./a.out.

It prints a lot of pairs of the form (large_integer, 1008), a couple of (0, 1008) and exits with "Segmentation fault".

Answer 1

The comparison predicate should return a bool: true if a < b and false otherwise. Change the return statement to:

  return a < b;

Not to be confused with C-style 3-way comparison functions.

NOTE: Though C++20 does introduce a 3-way comparison operator <=>, sort would still expect a 2-way compare predicate.