Question

Possible Duplicate:
In STL maps, is it better to use map::insert than []?

I was wondering, when I insert element into map, what is the recommended way. Should I

map[key] = value;

or

map.insert(std::pair<key_type, value_type>(key, value));

I did the following quick test:

#include <map>
#include <string>
#include <iostream>

class Food {
public:
    Food(const std::string& name) : name(name) { std::cout << "constructor with string parameter" << std::endl; }
    Food(const Food& f) : name(f.name) { std::cout << "copy" << std::endl; }
    Food& operator=(const Food& f) { name = f.name; std::cout << "=" << std::endl; return *this; } 
    Food() { std::cout << "default" << std::endl; }
    std::string name;
};

int main() {
    std::map<std::string, Food> m0;

/*
1) constructor with string parameter
2) copy
3) copy
4) copy
*/
    m0.insert(std::pair<std::string, Food>("Key", Food("Ice Cream")));

/*
1) constructor with string parameter
2) default
3) copy
4) copy
5) =
*/
    // If we do not provide default constructor.
    // C2512: 'Food::Food' : no appropriate default constructor available
    m0["Key"] = Food("Ice Cream");
}

1. I realize by using member function insert, less value’s function call will be involved. So, is using insert a recommended way?
2. Why default constructor is needed, when map[key] = value way is being used?

I know that insert doesn’t overwrite existence key value pair, but map[key] = value does. However, is this the only factor I take into consideration, when try to choose among the both?

How about

1. Performance
2. Availability of value’s default constructor
3. ???

Answer 1

1. insert is not a recommended way – it is one of the ways to insert into map. The difference with operator[] is that the insert can tell whether the element is inserted into the map. Also, if your class has no default constructor, you are forced to use insert.
2. operator[] needs the default constructor because the map checks if the element exists. If it doesn’t then it creates one using default constructor and returns a reference (or const reference to it).

Because map containers do not allow for duplicate key values, the insertion operation checks for each element inserted whether another element exists already in the container with the same key value, if so, the element is not inserted and its mapped value is not changed in any way.