Question

I understand that when we insert values into the STL map, a copy is made and stored.
I have code that essentially does a find on the map and obtains an iterator.

I then intend to use the iterator to change the value in the map.
The results are not what I would expect ie: the value is not changed when accessed from another part of the program. I suspect its because the change I am applying is to a copy of
value.
the relevant code is as follows.

ObjectMappingType::iterator it = objectMapping_.find(symbol);

if (it == objectMapping_.end()) {
    throw std::invalid_argument("Unknown symbol: " + symbol);
}
get<3>(it->second) = value;

NOTE: I am actually trying to change a value inside a boost::tuple that is stored as the ‘value’ part of the map.

Answer 1

Hmm… both methods seem to work fine for me. Here’s the entire example that I used:

#include <iostream>
#include <map>
#include <string>
#include <boost/tuple/tuple.hpp>

typedef boost::tuple<int, std::string> value_type;
typedef std::map<int, value_type> map_type;

std::ostream&
operator<<(std::ostream& os, value_type const& v) {
    os << " number " << boost::get<0>(v)
       << " string " << boost::get<1>(v);
    return os;
}

int
main() {
    map_type m;

    m[0] = value_type(0, "zero");
    m[1] = value_type(0, "one");
    m[2] = value_type(0, "two");

    std::cout
        << "m[0] " << m[0] << "\n"
        << "m[1] " << m[1] << "\n"
        << "m[2] " << m[2] << "\n"
        << std::endl;

    boost::get<0>(m[1]) = 1;

    map_type::iterator iter = m.find(2);
    boost::get<0>(iter->second) = 2;

    std::cout
        << "m[0] " << m[0] << "\n"
        << "m[1] " << m[1] << "\n"
        << "m[2] " << m[2] << "\n"
        << std::endl;

    return 0;
}

The output is exactly what I would have expected.

lorien$ g++ -I/opt/include -gdwarf-2 foo.cpp 
lorien$ ./a.out
m[0]  number 0 string zero
m[1]  number 0 string one
m[2]  number 0 string two

m[0]  number 0 string zero
m[1]  number 1 string one
m[2]  number 2 string two
lorien$