Question

Suppose I have the following snippets:

int compareFoo(std::string** a, std::string** b) {
    return (**a).compare(**b);
}

int main() {
    std::string** foo = new std::string*[3];
    foo[0] = new std::string("a");
    foo[1] = new std::string("c");
    foo[2] = new std::string("b");
    sort(foo,foo+3,compareFoo);
    for (int i = 0; i < 3; i++) {
        std::cout << *foo[i] << std::endl; // print acb
    }
}

If I’d left out the third parameter(compare) for sort, it’d have given me the sorted 3 strings in terms of their memory addresses, that’s not how I intended it. But how do I parameterize the compareFoo function so that it won’t compare the memory addresses.

void sort(RandomAccessIterator first, RandomAccessIterator last, Compare comp);

The description of Sort on cplusplus.com is quite vague and the given example is simple. Since it takes an Iterator, does it mean that i only work with stand containers? Thank you

Answer 1

The comparison function takes two items to compare and returns true if the first one is less than the second one. In your case, it would work like this:

#include <string>
#include <algorithm>
#include <iostream>

using std::sort;

bool compareFoo(std::string* a,std::string* b){
  return *a < *b;
}

int main(){
  std::string** foo = new std::string*[3];
  foo[0] = new std::string("a");
  foo[1] = new std::string("c");
  foo[2] = new std::string("b");
  sort(foo,foo+3,compareFoo);
  for(int i=0;i<3;i++){
    std::cout << *foo[i] << std::endl; 
  }

  // Remember to delete things that you dynamically allocate.
  delete foo[0];
  delete foo[1];
  delete foo[2];
  delete [] foo;
}