Question

I was playing with some code and I wrote this little class to pool std::wstring. As I profile it I’d like to know your oppinion about approaching a pool of strings this simple.

I have a std::vector of std::wstring and two maps, one for holding the indexes of the unused elements in the vector and another for the ones that are being used.

If a string is requested with new_string() and there isn’t any free indexes left, I resize the vector and insert the newly added indexes to the free indexes map.

Here is the code:

typedef std::wstring string_t;

class string_pool
{
    public:
        string_pool()
        {
            grow_factor = 2;
            strings.resize(10);
            for (size_t i=0; i<10; i++)
            {
                free_indexes.insert(std::make_pair(&strings[i],i));
            }
        }

        virtual ~string_pool()
        {

        }

        string_t* new_string()
        {
            if (!free_indexes.size())
            {
                size_t old_size = strings.size();
                size_t new_size = old_size+grow_factor;
                for (size_t i=old_size;i<new_size; i++)
                {
                    free_indexes.insert(std::make_pair(&strings[i],i));
                }
                strings.resize(new_size);
            }
            iter = free_indexes.begin();
            used_indexes.insert(*iter);     // Mark the index as used
            string_t* str = (*iter).first;    // Get a pointer to the string
            free_indexes.erase(iter);       // Erase the free index
            return str;
        }

        void delete_string(string_t* str)
        {
            str->clear();
            iter = used_indexes.find(str);
            free_indexes.insert(*iter);     // Mark the index as free
            used_indexes.erase(iter);       // Erase the used index
        }

    protected:
        std::map<string_t*, size_t>             used_indexes;
        std::map<string_t*, size_t>             free_indexes;
        std::vector<string_t>                   strings;
        size_t                                  grow_factor;
        std::map<string_t*, size_t>::iterator   iter;
};

Answer 1

The new/delete you care about is not allocation of the string descriptors, but the string data. Pooling string objects doesn’t help much, the content will still be dynamically allocated.