Getting the right value_type

Question

In my class I have a member:

std::vector<std::string> memory_;

Now I'd like to have a fnc returning what's in the memory's first element but I do not want to specify std::string as a return type in case later I decide to use different type for this purpose so I've tried this but it doesn't work:

typename decltype(memory_)::value_type call_mem()
{
    return memory_[0];
}

Any ideas how to specify return type in the most generic way?

Answer 1

As long as you use a standard container, that should work, and I think is okay.

Alternatively, since it is a member of a class, then you can use typedef and expose the value_type as nested type of the class:

class demo
{
   public:
     typedef std::vector<std::string> container_type;
     typedef container_type::value_type value_type;

     value_type call_mem()
     {
         return *std::begin(memory_); //it is more generic!
     }

   private:        
     container_type memory_;
};

Note that *std::begin(memory_) is more generic than both memory_[0] and *memory_.begin() as with it, even arrays would work, but that is less likely to benefit you in real code.

Answer 2

You actually just needed to change your formatting slightly, and use the auto keyword:

auto call_mem() -> decltype(memory_)::value_type
{
    return memory_[0];
}

Answer 3

Actually you could just decltype the whole expression:

decltype(memory_[0]) call_mem()
{
    return memory_[0];
}

But ensure that memory_ is declared before call_mem. (and use std::begin to generalize to other containers as explained by @Nawaz.)