Dynamically allocation for a map pointer value

Question

Say I have a class that contains a map with a pointer-type key:

class Base;
class App
{
  private:
    size_t n;
    map<string, Base*> m;
};

What I want is that, when I refer to a map key for the first time, I need the "Base*" pointer already allocated for "n" elements. I can't do this allocation in the constructor, because the map key value will only be known at run-time. Not sure what is the best solution for this.

Firstly, you mean "value", not "key", right? Secondly, how and when is n determined? Is it a compile-time constant? — jogojapan
– jogojapan, Commented Nov 10, 2013 at 2:23

LihO · Accepted Answer · 2013-11-10 02:30:01Z

1

"Not sure what is the best solution for this"

I would say that avoiding dynamically allocated C-style array would be a good start. Maybe instead of

map<string, Base*> m;

you could use:

map<string, std::vector<Base> > m;

"when I refer to a map key for the first time, I need the "Base*" pointer already allocated for "n" elements"

You could do something like this:

std::vector<Base>& getVal(const std::string& key) {
    if (m.count(key) > 0 && m[key].size() > 0)
        return m[key];
    m[key] = std::vector<Base>(10); // n
    return m[key];
}

answered Nov 10, 2013 at 2:30

LihO

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