How should std::map be used with a value that does not have a default constructor?

Question

I\'ve got a value type that I want put into a map. It has a nice default copy constructor, but does not have a default constructor.

I believe that so long as I stay

Answer 1

You can simply "insert-or-overwrite" with the standard insert function:

auto p = mymap.insert(std::make_pair(key, new_value));

if (!p.second) p.first->second = new_value;  // overwrite value if key already exists

If you want to pass the elements by rerference, make the pair explicit:

insert(std::pair(key, value));

If you have a typedef for the map like map_t, you can say std::pair, or any suitable variation on this theme.

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Maybe this is best wrapped up into a helper:

template 
void insert_forcefully(Map & m,
                       typename Map::key_type const & key,
                       typename Map::mapped_type const & value)
{
  std::pair p = m.insert(std::pair(key, value));
  if (!p.second) { p.first->second = value; }
}