Question

I am trying to implement the following method to get the bus corresponding to its name which is passed as a string to the method. Here AbstractBus is an abstract class which contains functionality of a bus. localBusses is a std::map

void getLocalBusByName(string& name, AbstractBus& bus) 
{
    bus =  localBusses.find(name)->second;
}

However, I am getting an error “no operator “=” matches these operands”
Can anyone explain what is wrong here and how can I correct it?

Answer 1

The code presents several problems

1. It does not handle the case when localBusses.find(…) doesn#t find a bus
2. You seem to want to return a reference to the found bus. You should remember that references are aliases and they can bind only when declared and remain immutable afterwards. That is you cannot re-bind a reference to another object
3. Following 2, the bus = … attempts to call the assignment operator to assign the found bus to the one refered to by the reference. The assignment operator does not exist or is not accessible. This is good so since having an assignment operator in this context is a bad idea and if present it would probably lead to slicing if the AbstractBus class would be instantiable (non-abstract).

EDIT: I believe you either have:

typedef std::map<string, AbstractBus*> tBussesMap;
tBussesMap localBusses;

or AbstractBus is not really abstract (does not contain any pure methods) (despite its name)

In either case I assume however you have classes derived from AbstractBus. And getLocalBusByName has to provide for situations where no bus is found and avoid inadvertent slicing.

So you should write getLocalBusByName method in the lines of:

AbstractBus* 
getLocalBusByName( const string& name )
{
    tBussesMap::iterator it = localBusses.find(name);

    if ( it != localBusses.end() )
    {
        return it->second;
    }

    return nullptr;
}

where localBusses is assumed to be of type:std::map<string, AbstractBus*> (the only viable way if AbstractBus is abstract)