map<string, string> dada; dada["dummy"] = "papy"; cout << dada["pootoo"]; I'm puzzled because I don't know if it's considered undefined behaviour or not, how to know when I request a key which does not exist, do I just use find instead ?
Add a comment |
7 Answers
The map::operator[] searches the data structure for a value corresponding to the given key, and returns a reference to it.
If it can't find one it transparently creates a default constructed element for it. (If you do not want this behaviour you can use the map::at function instead.)
You can get a full list of methods of std::map here:
http://en.cppreference.com/w/cpp/container/map
Here is the documentation of map::operator[] from the current C++ standard...
23.4.4.3 Map Element Access
T& operator[](const key_type& x);
Effects: If there is no key equivalent to x in the map, inserts value_type(x, T()) into the map.
Requires: key_type shall be CopyConstructible and mapped_type shall be DefaultConstructible.
Returns: A reference to the mapped_type corresponding to x in *this.
Complexity: logarithmic.
T& operator[](key_type&& x);
Effects: If there is no key equivalent to x in the map, inserts value_type(std::move(x), T()) into the map.
Requires: mapped_type shall be DefaultConstructible.
Returns: A reference to the mapped_type corresponding to x in *this.
Complexity: logarithmic.
2 Comments
Vlad Ciobanu
I actually did not find anywhere in the standard that it says that. At the very least, not at 23.3.1.2 [lib.map.access] or anywhere else that I could remember to check. EDIT: Apparently I was looking at an old version of the standard. My mistake.
Andrew Tomazos
@VladCiobanu: Above is 23.4.4.3 copied verbatim. Are you using the current version
ISO/IEC 14882:2011(E)?If you try to access a key value using index operator [], then 2 things can happen :
- The map contains this
key. So it will return the correspondingkey value. - The map doesn't contain the
key. In this case, it will automatically add akeyto the map withnull value.
"pootoo" key does't exist in your map. So it will automatically add this key with value = ""(empty string). And your program will print empty string.
Here map size will increase by 1.
To search a key you can use map_name.find(), which will return map_name.end() if the key doesn't exist. And no extra key will be added.
You can use [] operator when you want to set value for a key.
Comments
It's not undefined behavior. If operator [] doesn't find a value for the provided key, it inserts one at that position.
1 Comment
SRIDHARAN
helpful. "it inserts one at that position." - it increases the size of map by 1 if it doesn't exist.
For operator[], if you try to access a value for a key that doesn't exist, a new value object that has been default constructed will be put into the map and it's reference returned.
Comments
The operator[] for map returns a non-const reference and you can assign using that in the way you've shown on your second line. Accessing in this way will create a default contructed element of value type.
If you want to find a find an element, a better way is
iterator find ( const key_type& x ) (or the const alternative) which will return an iterator equal to <map>.end() if it doesn't find the key, or if you just want to know if it's in the collection you can use
size_type count ( const key_type& x ) const which will always return either 1 or 0 for a map since keys are unique.
Comments
If operator [] doesn't find a value for the provided key, it inserts one at that position.
But you should note that if you visit a not exist key and invoke it's member function, like mapKV[not_exist_key].member_fun().The program may crash.
Let me give an example, test class as below:
struct MapValue{ int val; MapValue(int i=0){ cout<<"ctor: "<<i<<endl; val = i; } ~MapValue(){ cout<<"dtor: "<<val<<endl; } friend ostream& operator<<(std::ostream& out, const MapValue& mv){ cout<<"MapValue: "<<mv.val<<endl; } string toString(){ cout<<"MapValue: "<<val<<endl; } }; Test code:
cout<<"-------create map<int, MapValue>-------"<<endl; map<int, MapValue> idName{{1, MapValue(1)}, {2, MapValue(2)}}; cout<<"-----cout key[2]-----"<<endl; cout<<idName[2]<<endl; cout<<"-----cout key[5]-----"<<endl; cout<<idName[5]<<endl; cout<<"------- runs here means, does't crash-------"<<endl; Output as below:
-------create map<int, MapValue>------- ctor: 1 ctor: 2 dtor: 2 dtor: 1 dtor: 2 dtor: 1 -----cout key[2]----- MapValue: 2 -----cout key[5]----- ctor: 0 MapValue: 0 -------runs here means, does't crash------- dtor: 0 dtor: 2 dtor: 1 We can see that: idName[5] invoke map construct {5, MapValue(0)} to insert to idName.
But if, you invoke member function by idName[5], then the program crashes :
cout<<"-------create map<int, MapValue>-------"<<endl; map<int, MapValue> idName{{1, MapValue(1)}, {2, MapValue(2)}}; idName[5].toString(); // get crash here. cout<<"------- runs here means, doesn't crash-------"<<endl; 
1 Comment
isti_spl
it shouldn't crash because the map, must be some other mistake
please have a look at the out_of_range exception: http://www.cplusplus.com/reference/stdexcept/out_of_range/
this is what map::at and map::operator[] will throw if key does not exist. You can catch it the same way as the vector example in the link.
You can also use: http://www.cplusplus.com/reference/map/map/find/
And check against map::end
1 Comment
galka
no, only map::at throws, map::operator[] creates a new value_type if key doesn't exist