insert to a pointer to std::map

Question

I'm using a map inside a map:

std::map<int, std::map<DWORD,IDLL::CClass*>*> *MyMap

I'm using the following code to insert into the map:

std::map<DWORD,IDLL::CClass*> *SecondMap;
SecondMap= new std::map<DWORD,IDLL::CClass*>; 
DWORD i = 1;
while (eteration on obj type IDLL::CClass ) 
{
     SecondMap.insert(std::make_pair(i,&obj));           
}int j = 1;    


MyMap->insert(std::make_pair(1,&SecondMap));

Later in the programm the map has become empty!

Could any one help please?

What is the reason not to use std::map<int, std::map<DWORD,IDLL::CClass> > MyMap (without the pointers)? — ipc
– ipc, Commented Mar 12, 2013 at 17:21
I have to use many maps so I choose poniters to decrease the memory use — user2161341
– user2161341, Commented Mar 12, 2013 at 17:25
@user2161341: unless there are mulitple pointers to common maps, using pointers will increase the memory use. Since it looks like you create a new map for every pointer, there will be no common maps. — Chris Dodd
– Chris Dodd, Commented Mar 12, 2013 at 17:26
std::map<std::pair<int, DWORD>,IDLL::CClass*> my_map may work better for you. You can simple combine the two keys with a std::pair, and stop using pointers while your at it. Containers do not generally need to be declared on the heap. — andre
– andre, Commented Mar 12, 2013 at 17:54

Mr.C64 · Accepted Answer · 2013-03-12 18:05:45Z

Having owning raw pointers is a bad practice in general (unless you are in special cases, like if you are building a custom advanced data structures, and the owning raw pointers are protected in proper RAII class boundaries).

The first option should be to use value semantics; if that is not good for your performance, then you may want to use smart pointers like shared_ptr (this helps make your code simpler, make it exception-safe, avoid resource leaks, etc.).

This is a sample compilable code, that seems to work on VS2010 SP1 (VC10):

#include <iostream>     // for std::cout
#include <map>          // for std::map
#include <memory>       // for std::shared_ptr and std::make_shared
using namespace std;

typedef unsigned int DWORD;    

struct MyClass
{
    MyClass() : Data(0) {}
    explicit MyClass(int data) : Data(data) {}

    int Data;
};

int main()
{
    //
    // Build some test maps
    //

    typedef map<DWORD, shared_ptr<MyClass>> InnerMap;
    typedef map<int, shared_ptr<InnerMap>> OuterMap;

    auto innerMap1 = make_shared<InnerMap>();
    (*innerMap1)[10] = make_shared<MyClass>(10);
    (*innerMap1)[20] = make_shared<MyClass>(20);

    OuterMap myMap;

    myMap[30] = innerMap1;


    //
    // Testing code for maps access
    //

    const int keyOuter = 30;
    auto itOuter = myMap.find(keyOuter);
    if (itOuter != myMap.end())
    {
        // Key present in outer map.
        // Repeat find in inner map.

        auto innerMapPtr = itOuter->second;
        const DWORD keyInner = 20;
        auto itInner = innerMapPtr->find(keyInner);
        if (itInner !=  innerMapPtr->end())
        {
            cout << itInner->second->Data << '\n';
        }
    }
}

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