Question

A friend sent me this and I really don’t know what it is, inside the loop. Whatever it be it calls the std::set constructor half million times…. any help appreciated. I would expect a compiler error, but it actually compiles in g++ 4.4 and 4.5 and the behavior is different to copy construction…

#include <stdio.h>
#include <stdlib.h>
#include <boost/unordered_map.hpp>
#include <set>
#include <string>

typedef boost::unordered_map<int, std::set<int> > mymap;

int main () {
    mymap map;
    for ( int i = 0 ; i < 1000 ; i++ )     
    {
        std::set<int>  map[i] ;
    }
    return 1;
};

Answer 1

You are dealing with a GCC-specific non-standard extension of C++ language. Each iteration defines an array of std::map objects with i elements (and immediately destroys it).

In standard C++ it is illegal to use a non-constant expression to specify array size, so the code is not legal C++. It compiles, again, only because GCC allows it as an extension.