c++ - How to insert key and value to map that is inside another map in cpp? -

i have map :

std::map<std::string, std::map<std::string, std::string> > objects; 

as far know, when write objects[key] replace value of key exists, here want add second map if pair doesn't exist.

example:      current map :("key1",map(<"key_a","value_a">,<"key_c","value_d">))  input:  "key1", "key_e","value_f"  relsult: ("key1",map(<"key_a","value_a">,<"key_c","value_d">,<"key_e","value_f">))  input : "key1", "key_e","value_g"  (replace value "value_f" "value_g") result: ("key1",map(<"key_a","value_a">,<"key_c","value_d">,<"key_e","value_g">))  , if "key2" -> insert new key empty map 

can :

objects[key].insert(std::make_pair(key2,value2)) 

it didn't work reason

can please advise?

as far know, when write objects[key] replace value of key exists, here want add second map if pair doesn't exist.

that's wrong: if write objects[key] doesn't replace anything, ever; rather:

• if objects doesn't contain key, inserts new pair in map key , value uses default constructed (in case empty) std::map<std::string, std::string>. returns reference value.

• if objects contain key, returns reference existing value (the "inner" map) without changing or replacing anything.

can this: objects[key].insert(std::make_pair(key2,value2))

you could, if [key][key2] existing wouldn't update value value2, assume want , why "it didn't work".

all have is:

objects[key][key2] = value2; 

because objects[key] never replaces existing map, default-constructs 1 if key's missing, key2 either finds existing entry in "inner" map needs updating or creates it, hangs (if i've understood requirement correctly).