how to overite multimap::insert?

[Olivier Planchon]

Hi,

I am coding a multithread program.
I have a multimap that will receive data from many threads.
Therefore I want to wrap the class in a derived class with overwitten members as necessary.
This is my code to do his:

template <typename T> class Border_map :
    public std::multimap<T, RC>
{
    std::mutex _mutex ;

public:
    std::multimap<T, RC>::iterator insert ( const value_type& x ) // ERROR HERE
    {
        std::lock_guard<std::mutex> guard (_mutex) ;
        return std::multimap<T, RC>::insert(x) ;
    }
} ;

The compiler does not understand any of my attempts at describing the return type of the function insert. What should I write instead?

thanks, Olivier

[Cubbi]

First of all, what is "RC"? It's not declared anywhere in your example.

Second, both iterator and value_type are dependent types, the correct syntax would be
typename std::multimap<T, RC>::iterator insert(const typename std::multimap<T, RC>::value_type& x )

Third, don't inherit from std::multimap, use composition.

[LB]

iterator is already in scope because you inherited.

[Olivier Planchon]

@Cubbi
Thanks. it works. I was not familiar with this use of keyword typename. As far as I understood what I read since your reply, we must use it anywhere the compiler cannot understand from te context that we are actually describing a type name. Correct?

why did you advise not to inherit from std::multimap?

I chose heritage because I did not want to overwrite all the methods I will be using, but only those for which I need a lock. Is there anything wrong in this logic? or is there something more important that I have ignored?

@L B
I thought it was in the scope, but the compiler did not accept a line like this:iterator insert(...)

[Cubbi]

why did you advise not to inherit from std::multimap?

Because it is not a polymorphic class. Specifically, its destructor is public nonvirtual. While you technically can do this, public inheritance from such classes is bad design.