std::map operator[]

since C++11

// (1) Non const version only
T& operator[]( const Key& key );
// (2) Non const version only
T& operator[]( Key&& key );

since C++98

// (1) Non const version only
T& operator[]( const Key& key );

Returns a reference to the value that is mapped to a key equivalent to key, performing an insertion if such key does not already exist.

since C++11

(1) Inserts a value_type object constructed in-place from std::piecewise_construct, std::forward_as_tuple(key), std::tuple<>() if the key does not exist.

Since C++17 this function is equivalent to return this->try_emplace(key).first->second.

When the default allocator is used, this results in the key being copy constructed from key and the mapped value being value-initialized.

Type requirements

• value_type must be EmplaceConstructible from std::piecewise_construct, std::forward_as_tuple(key), std::tuple<>(). When the default allocator is used, this means that key_type must be CopyConstructible and mapped_type must be DefaultConstructible.

(2) Inserts a value_type object constructed in-place from std::piecewise_construct, std::forward_as_tuple(std::move(key)), std::tuple<>() if the key does not exist.

Since C++17 this function is equivalent to return this->try_emplace(key).first->second.

When the default allocator is used, this results in the key being move constructed from key and the mapped value being value-initialized.

Type requirements

• value_type must be EmplaceConstructible from std::piecewise_construct, std::forward_as_tuple(std::move(key)), std::tuple<>(). When the default allocator is used, this means that key_type must be MoveConstructible and mapped_type must be DefaultConstructible.

until C++11

(1) Inserts value_type(key, T()) if the key does not exist. This function is equivalent to return insert(std::make_pair(key, T())).first->second If an insertion is performed, the mapped value is value-initialized (default-constructed for class types, zero-initialized otherwise) and a reference to it is returned.

Type requirements

• key_type must meet the requirements of CopyConstructible.
• mapped_type must meet the requirements of CopyConstructible and DefaultConstructible.

Parameters

• key - the key of the element to find

Return value

Reference to the mapped value of the new element if no element with key key existed. Otherwise a reference to the mapped value of the existing element whose key is equivalent to key.

Exceptions

If an exception is thrown by any operation, the insertion has no effect.

Complexity

Logarithmic in the size of the container.

Example

Main.cpp

#include <iostream>
#include <map>

auto main() -> int {
	std::map<std::string, int> playersScores = {
		{ "player1", 50 },
		{ "super_coder", 100 },
		{ "andrew111", 41 }
	};

	std::cout << "player1 zdobył " << playersScores["player1"] << " punktów.\n";

	std::cout << "super_cader zdobył " << playersScores["super_cader"] << " punktów.\n";
}

Output

player1 zdobył 50 punktów.
super_cader zdobył 0 punktów.