What happens if you call erase() on a map element while iterating from begin to end?

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C++11

This has been fixed in C++11 (or erase has been improved/made consistent across all container types).
The erase method now returns the next iterator.

auto pm_it = port_map.begin();
while(pm_it != port_map.end())
{
    if (pm_it->second == delete_this_id)
    {
        pm_it = port_map.erase(pm_it);
    }
    else
    {
        ++pm_it;
    }
}

C++03

Erasing elements in a map does not invalidate any iterators.
(apart from iterators on the element that was deleted)

Actually inserting or deleting does not invalidate any of the iterators:

Also see this answer:
Mark Ransom Technique

But you do need to update your code:
In your code you increment pm_it after calling erase. At this point it is too late and is already invalidated.

map<string, SerialdMsg::SerialFunction_t>::iterator pm_it = port_map.begin();
while(pm_it != port_map.end())
{
    if (pm_it->second == delete_this_id)
    {
        port_map.erase(pm_it++);  // Use iterator.
                                  // Note the post increment.
                                  // Increments the iterator but returns the
                                  // original value for use by erase 
    }
    else
    {
        ++pm_it;           // Can use pre-increment in this case
                           // To make sure you have the efficient version
    }
}

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