An element can be inserted into a std::map only if its key is not already present in the map. Given for example:

std::map< std::string, size_t > fruits_count;
fruits_count.insert({"grapes", 20});
fruits_count.insert(make_pair("orange", 30));
fruits_count.insert(pair<std::string, size_t>("banana", 40));
fruits_count.insert(map<std::string, size_t>::value_type("cherry", 50));

The insert() function returns a pair consisting of an iterator and a bool value:

The following method can be used to combine insertion and searching operation:

auto success = fruits_count.insert({"grapes", 20});
if (!success.second) {           // we already have 'grapes' in the map
    success.first->second += 20; // access the iterator to update the value
fruits_count["apple"] = 10;

While simpler, it prevents the user from checking if the element already exists. If an element is missing, std::map::operator[] implicitly creates it, initializing it with the default constructor before overwriting it with the supplied value.

fruits_count.insert({{"apricot", 1}, {"jackfruit", 1}, {"lime", 1}, {"mango", 7}});
std::map< std::string, size_t > fruit_list{ {"lemon", 0}, {"olive", 0}, {"plum", 0}};
fruits_count.insert(fruit_list.begin(), fruit_list.end());


std::map<std::string, size_t> fruits_count;
std::string fruit;
while(std::cin >> fruit){
    // insert an element with 'fruit' as key and '1' as value
    // (if the key is already stored in fruits_count, insert does nothing)
    auto ret = fruits_count.insert({fruit, 1});
    if(!ret.second){            // 'fruit' is already in the map 
        ++ret.first->second;    // increment the counter

Time complexity for an insertion operation is O(log n) because std::map are implemented as trees.

A pair can be constructed explicitly using make_pair() and emplace():