How to free memory used by boost::multi_index::multi_index_container?

Question

How to free the memory used by the "person_map_t multi_indexed_persons_map"? I cant find something about it on google.

This is my testcode:

#include <sstream>
#include <ostream>
#include <stdio.h>
#include <iostream>
#include <string>
#include <boost/lexical_cast.hpp>
#include <boost/multi_index_container.hpp>
#include <boost/multi_index/hashed_index.hpp>
#include <boost/multi_index/mem_fun.hpp>
#include <boost/multi_index/tag.hpp>
#include <boost/multi_index/ordered_index.hpp>

struct name {};
struct age {};

struct person
{
    std::string name_;
    int age_;

    std::string name() const
    {
        return name_;
    }

    int age() const
    {
        return age_;
    }
};

typedef boost::multi_index::multi_index_container<
    person,
    boost::multi_index::indexed_by<
        boost::multi_index::ordered_unique<
            boost::multi_index::tag<name>,
            boost::multi_index::const_mem_fun<person, std::string, &person::name>
        >,
        boost::multi_index::ordered_non_unique<
            boost::multi_index::tag<age>,
            boost::multi_index::const_mem_fun<person, int, &person::age>
        >
    >
> person_map_t;

int main(int argc, char *argv[])
{
    std::string userinput;

    {
        person_map_t multi_indexed_persons_map;
        for (int i = 0; i < 100000; i++)
        {
            person p;
            p.name_ = std::string("MyName_") + boost::lexical_cast<std::string>(i);
            p.age_ = i;
            multi_indexed_persons_map.insert(p);
        }

        std::cout << "Done inserting." << std::endl;

        std::cin >> userinput; // while this is blocking, check consumtion with: `ps u -C <binname>`
    }

    // multi_indexed_persons_map.erase(multi_indexed_persons_map.begin(), multi_indexed_persons_map.end()); /* dosnt work too ... */

    std::cout << "Memory freed?" << std::endl;
    // out of scope, memory should be freed now,
    // check again with: `ps u -C <binname>` and compare.
    std::cin >> userinput;

    return 0;
}

How to test:

1. Start the binary.
2. Wait till its done inserting.
3. Check memory consumption with ps u -C <binname>.
4. cin somthing, at least one sign.
5. Check memory consumption again with ps u -C <binname>.

It now should be freed some memory, right?

EDIT:

valgrind output:

==2314== 
==2314== HEAP SUMMARY:
==2314==     in use at exit: 0 bytes in 0 blocks
==2314==   total heap usage: 400,005 allocs, 400,005 frees, 16,489,069 bytes allocated
==2314== 
==2314== All heap blocks were freed -- no leaks are possible
==2314== 
==2314== For counts of detected and suppressed errors, rerun with: -v
==2314== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 6 from 6)

Answer 1

It should and I guess it will, although I don't know the details of multi_index_container. This is more related to the platform you use than to anything regarding boost or C++.

If you put logging in the Person's ctors and dtors you can watch whether Persons are still around, I suspect they won't.

Whether the process actually returned memory to the operating system depends on the platform, but there usually is a layer of memory management between the language itself and the operating system that can hold on to memory I believe.

Also, as Emile pointed out, stl and boost classes use allocators that are an additional layer above the runtime that can also hold on to memory.

Answer 2

Memory freeing happens when it's out of scope and not when you use erase. To get it out of scope surround it and the code that uses it with { } and after the } the memory will be freed.