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std::{{{1}}}::operator[]

من cppreference.com
T& operator[]( const Key& key );
(1) قالب:mark since none
T& operator[]( Key&& key );
(2) (since C++11)

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.

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.

1) Inserts value_type(key, T()) قالب:par req conceptقالب:par req concept
2) Inserts value_type(std::move(key), T()) قالب:par req conceptقالب:par req concept

Information on iterator invalidation is copied from here

محتويات

[تعديل] Parameters

key - the key of the element to find

[تعديل] 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.

[تعديل] Exceptions

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

[تعديل] Complexity

قالب:included


[تعديل] Example

This example demonstrates how to modify existing values and insert new values using ‎operator[]‎:

#include <iostream>
#include <>
 
int main()
{
    std::<char, int> letter_counts {{'a', 27}, {'b', 3}, {'c', 1}};
 
    std::cout << "initially:\n";
    for (const auto &pair : letter_counts) {
        std::cout << pair.first << ": " << pair.second << '\n';
    }
 
    letter_counts['b'] = 42;  // update an existing value
 
    letter_counts['x'] = 9;  // insert a new value
 
    std::cout << "after modifications:\n";
    for (const auto &pair : letter_counts) {
        std::cout << pair.first << ": " << pair.second << '\n';
    }
}

الخرج:

initially:
a: 27
b: 3
c: 1
after modifications:
a: 27
b: 42
c: 1
x: 9

The following example counts the occurrences of each word in a vector of strings:

#include <string>
#include <iostream>
#include <vector>
#include <>
 
int main()
{
    std::vector<std::string> words = { 
        "this", "sentence", "is", "not", "a", "sentence",
	"this", "sentence", "is", "a", "hoax" 
    };
 
    std::<std::string, size_t>  word_map;
    for (const auto &w : words) {
        ++word_map[w];
    }
 
    for (const auto &pair : word_map) {
        std::cout << pair.second
	          << " occurrences of word '"
	          << pair.first << "'\n";
    }
}

الخرج:

1 occurrences of word 'hoax'
2 occurrences of word 'this'
2 occurrences of word 'a'
2 occurrences of word 'is'
1 occurrences of word 'not'
3 occurrences of word 'sentence'

[تعديل] See also

قالب:dsc mem fun