std::map<Key,T,Compare,Allocator>::operator[]

Since C++11

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

# Declarations

T& operator[]( const Key& key );

T& operator[]( Key&& key );

(since C++11)

template< class K >
T& operator[]( K&& x );

(since C++26)

# Parameters

key: the key of the element to find
x: a value of any type that can be transparently compared with a key

# Notes

In the published C++11 and C++14 standards, this function was specified to require mapped_type to be DefaultInsertable and key_type to be CopyInsertable or MoveInsertable into *this. This specification was defective and was fixed by LWG issue 2469, and the description above incorporates the resolution of that issue.

However, one implementation (libc++) is known to construct the key_type and mapped_type objects via two separate allocator construct() calls, as arguably required by the standards as published, rather than emplacing a value_type object.

operator[] is non-const because it inserts the key if it doesn’t exist. If this behavior is undesirable or if the container is const, at may be used.

insert_or_assign returns more information than operator[] and does not require default-constructibility of the mapped type.

# Example

#include <iostream>
#include <string>
#include <map>
 
void println(auto const comment, auto const& map)
{
    std::cout << comment << '{';
    for (const auto& pair : map)
        std::cout << '{' << pair.first << ": " << pair.second << '}';
    std::cout << "}\n";
}
 
int main()
{
    std::map<char, int> letter_counts{{'a', 27}, {'b', 3}, {'c', 1}};
 
    println("letter_counts initially contains: ", letter_counts);
 
    letter_counts['b'] = 42; // updates an existing value
    letter_counts['x'] = 9;  // inserts a new value
 
    println("after modifications it contains: ", letter_counts);
 
    // count the number of occurrences of each word
    // (the first call to operator[] initialized the counter with zero)
    std::map<std::string, int>  word_map;
    for (const auto& w : {"this", "sentence", "is", "not", "a", "sentence",
                          "this", "sentence", "is", "a", "hoax"})
        ++word_map[w];
    word_map["that"]; // just inserts the pair {"that", 0}
 
    for (const auto& [word, count] : word_map)
        std::cout << count << " occurrence(s) of word '" << word << "'\n";
}

# Defect reports

DR	Applied to	Behavior as published	Correct behavior
LWG 334	C++98	the effect of overload (1) was simply returning(*((insert(std::make_pair(x, T()))).first)).second	provided its owndescription instead