std::map default value

Question

Is there a way to specify the default value std::map 's operator[] returns when an key does not exist?

Thanks!

Answer 1

There is no way to specify the default value - it is always valeu constructed by the default (zero parameter constructor).

In fact operator[] probably does more than you expect as if a value does not exist for the given key in the map it will insert a new one with the value from the default constructor.

Answer 2

Maybe you can give a custom allocator who allocate with a default value you want.

template < class Key, class T, class Compare = less<Key>,
       class Allocator = allocator<pair<const Key,T> > > class map;

Answer 3

The C++ standard (23.3.1.2) specifies that the newly inserted value is default constructed, so map itself doesn't provide a way of doing it. Your choices are:

• Give the value type a default constructor that initialises it to the value you want, or
• Wrap the map in your own class that provides a default value and implements operator[] to insert that default.

Answer 4

No, there isn't. The simplest solution is to write your own free template function to do this. Something like:

#include <string>
#include <map>
using namespace std;

template <typename K, typename V>
V GetWithDef(const  std::map <K,V> & m, const K & key, const V & defval ) {
   typename std::map<K,V>::const_iterator it = m.find( key );
   if ( it == m.end() ) {
      return defval;
   }
   else {
      return it->second;
   }
}

int main() {
   map <string,int> x;
   ...
   int i = GetWithDef( x, string("foo"), 42 );
}