Question

The following code

#include "stdafx.h"
#include <string>
#include <set>
#include <boost/algorithm/string/trim.hpp>

int _tmain(int argc, _TCHAR* argv[])
{
    std::set<std::string> test;

    test.insert("test1   ");
    test.insert("test2 ");

    for(std::set<std::string>::iterator iter = test.begin(); iter != test.end(); ++iter)
    {
        boost::algorithm::trim(*iter);
    }

    return 0;
}

compiles in VS2008 but fails in VS2010 with the error

error C2663: 'std::basic_string<_Elem,_Traits,_Ax>::erase' : 3 overloads have no legal conversion for 'this' pointer    \boost\include\boost\algorithm\string\trim.hpp  

which indicates there is a problem with const matching in the functions. If I change set to vector everything is fine. I can also do a

boost::algorithm::trim(const_cast<std::string&>(*iter));

but I hate putting that in my code, and it seems like I shouldn’t have to as I’m not using a const iterator on the set. Does anyone have any ideas if this is the intended behavior and why?

Answer 1

The elements of a std::set are intended to be immutable. If you could update them in place, it would either require the set to be re-ordered whenever you updated an element (which would be very hard, possibly impossible, to implement), or updating an element would break the set’s guarantee of ordering.

Allowing the elements of a set to be mutable was an oversight in the original C++98 standard. It was corrected in C++11; the new standard requires set iterators to dereference to a const element. VS10 implements the new rule; VS08 followed the old standard, where updating a set element in place invokes undefined behaviour.

(See the final draft C++11 standard, section 23.2.4 para 6.)