std::map question on C++

I've a incertitude about std::map on c++:

I did a Object C_Configuration that loads a linked library (.so) C_ConfigurationLibrary.

The C_Configuration class has a std::map and The C_ConfigurationLibrary has a method that initializes the std::map.

If I access to the std::map from C_Configuration with a "for" loop:

std::map<const char*, const char*开发者_StackOverflow中文版>::iterator l_item;
for(l_item = m_configuration_map.begin();
    l_item != m_configuration.end();
    l_item++)

This is OK;

But if I use :

m_configuration[VALUE_KEY] // the value is NULL

This is not OK;

My CODE:

C_Configuration::C_Configuration()
{
    m_configuration = LoadLibrary(); // load the linked library (.so)

    if(m_configuration != NULL)
    {
        // DEBUG
        LOG_DEBUG("Loading Key from plugin...");

        m_configuration->LoadKeys(m_configuration_map);

        std::map <const char*, const char*>::iterator l_item;
        for ( l_item = l_configuration_map.begin();
              l_item != l_configuration_map.end();
              l_item++ )
        {                
            //THIS IS OK             
        }

        m_configuration_map[FIRST_KEY] // THIS IS NOT OK
    }
}

void C_ConfigLibrary::LoadKeys(std::map<const char*, const char*>& p_configuration_map)
{
    // DEBUG
    LOG_DEBUG("Loading Keys...");

    p_configuration_map.insert ( std::make_pair<const char*, const char*>(FIRST_KEY, FIRST_VALUE) );
    // DEBUG
    LOG_DEBUG("Loaded Key DBUS used: %s",m_dbus_used.c_str());
    p_configuration_map.insert ( std::make_pair<const char*, const char*>(SECOND_KEY,SECOND_VALUE) );
}

Can you help me?

thanks so much

You're using const char*s as keys, but pointers are compared on their memory addresses and not the text to which they point. Consequently, when you have a string literal in the shared library, and the same text in a string literal in the main app object, they can have different addresses and will not compare equal as keys.

You're much better off using std::strings as the keys, although it's safe to use const char* as the values.

FWIW, if you use const char* as a key, not only are string literals from different translation units sometimes finicky like this, but you'll have difficultly using text from local buffers, even std::string's .c_str() return values - it's just a very bad idea.

Does FIRST_KEY actually exists, or are you just using it as an example to show that you want the element at that given key?

The chances are that it doesn't work because you're storing C-strings as keys to your std::map. They're compared by their respective addresses, instead of the actual contents of the strings, which is likely the reason why it fails. You should be using std::string as a key to the map.

You should post the actual code though, otherwise you can only expect guess work. It's obvious that this piece of code is not the whole thing, because you're using FIRST_KEY etc. without us knowing what they are, and you have a mismatching number of brackets, as well as an incomplete statement that would produce an error.