Question

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*>::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

Answer 1

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.