number of elements in the table.
StringMap!double map; assert(map.length == 0); map["a"] = 3.0; assert(map.length == 1); map["c"] = 4.0; assert(map.length == 2); assert(map.remove("c")); assert(map.length == 1); assert(!map.remove("c")); assert(map.length == 1); assert(map.remove("a")); assert(map.length == 0);