Summary of C++ map access elements empty, size, max_size, operator[], at, insert, erase, swap, clear, emplace, emplace_hint (206)

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1. Preface

The purpose of this article: Understand the usage of map access elements empty, size, max_size, operator[], at, insert, erase, swap, clear, emplace, and emplace_hint in C++.

• 1. empty(): Check whether map is empty. If map is empty, return true; otherwise, return false.
• 2. size(): Get the number of key-value pairs in map.
• 3. max_size(): Gets the maximum number of elements that map may contain.
• 4. operator[]: Access or modify the value corresponding to a given key. If the key does not exist, the key is inserted into the map and associated with a default constructed value.
• 5. at(): Access the value corresponding to a given key. Similar to operator[], but if the key does not exist, a std::out_of_range exception will be thrown.
• 6. insert(): Insert a key-value pair into map.
• 7. erase(): Delete one or more key-value pairs from map.
• 8. swap(): Exchange the contents of two map.
• 9. clear(): Remove all key-value pairs from map, making it an empty container.
• 10. emplace(): Constructs a new key-value pair in place in map.
• 11. emplace_hint(): Constructs a new key-value pair in-place at the hint position at the given position.

2. Application examples

1. empty() function: Check whether map is empty. If map is empty, returns true; otherwise, returns false.

#include <iostream>
#include <map>

int main() {<!-- -->
    std::map<int, std::string> myMap;
    
    if (myMap.empty()) {<!-- -->
        std::cout << "Map is empty" << std::endl;
    } else {<!-- -->
        std::cout << "Map is not empty" << std::endl;
    }

    return 0;
}

2. size() function: Get the number of key-value pairs in map.

#include <iostream>
#include <map>

int main() {<!-- -->
    std::map<int, std::string> myMap = {<!-- -->{<!-- -->1, "apple"}, {<!-- -->2, "banana" }, {<!-- -->3, "orange"}};
    
    std::cout << "Size of map: " << myMap.size() << std::endl;

    return 0;
}

3. max_size() function: Get the maximum number of elements that map may contain.

#include <iostream>
#include <map>

int main() {<!-- -->
    std::map<int, std::string> myMap;
    
    std::cout << "Max size of map: " << myMap.max_size() << std::endl;

    return 0;
}

4. operator[]: Access or modify the value corresponding to a given key. If the key does not exist, the key is inserted into the map and associated with a default-constructed value.

#include <iostream>
#include <map>

int main() {<!-- -->
    std::map<char, int> myMap;
    
    myMap['a'] = 1;
    myMap['b'] = 2;
    
    std::cout << "Value of key 'a': " << myMap['a'] << std::endl;
    std::cout << "Value of key 'c': " << myMap['c'] << std::endl;

    return 0;
}

5. at() function: access the value corresponding to a given key. Similar to operator[], but if the key does not exist, a std::out_of_range exception will be thrown.

#include <iostream>
#include <map>

int main() {<!-- -->
    std::map<char, int> myMap = {<!-- -->{<!-- -->'a', 1}, {<!-- -->'b', 2}};
    
    try {<!-- -->
        std::cout << "Value of key 'a': " << myMap.at('a') << std::endl;
        std::cout << "Value of key 'c': " << myMap.at('c') << std::endl;
    } catch(const std::out_of_range & amp; e) {<!-- -->
        std::cout << "Key not found in map" << std::endl;
    }

    return 0;
}

6. insert() function: Insert a key-value pair into map.

#include <iostream>
#include <map>

int main() {<!-- -->
    std::map<int, std::string> myMap;
    
    myMap.insert({<!-- -->1, "apple"});
    myMap.insert({<!-- -->2, "banana"});
    myMap.insert({<!-- -->3, "orange"});
    
    std::cout << "Size of map: " << myMap.size() << std::endl;

    return 0;
}

7. erase() function: delete one or more key-value pairs from map.

#include <iostream>
#include <map>

int main() {<!-- -->
    std::map<int, std::string> myMap = {<!-- -->{<!-- -->1, "apple"}, {<!-- -->2, "banana" }, {<!-- -->3, "orange"}};
    
    myMap.erase(2);
    
    std::cout << "Size of map: " << myMap.size() << std::endl;

    return 0;
}

8. swap() function: exchange the contents of two map.

#include <iostream>
#include <map>

int main() {<!-- -->
    std::map<int, std::string> firstMap = {<!-- -->{<!-- -->1, "apple"}, {<!-- -->2, "banana" }};
    std::map<int, std::string> secondMap = {<!-- -->{<!-- -->3, "orange"}, {<!-- -->4, "grape" }};
    
    firstMap.swap(secondMap);
    
    std::cout << "Size of first map: " << firstMap.size() << std::endl;
    std::cout << "Size of second map: " << secondMap.size() << std::endl;

    return 0;
}

9. clear() function: Remove all key-value pairs from map, making it an empty container.

#include <iostream>
#include <map>

int main() {<!-- -->
    std::map<int, std::string> myMap = {<!-- -->{<!-- -->1, "apple"}, {<!-- -->2, "banana" }, {<!-- -->3, "orange"}};
    
    myMap.clear();
    
    std::cout << "Size of map after clearing: " << myMap.size() << std::endl;

    return 0;
}

10. emplace() function: Construct a new key-value pair in place in map.

#include <iostream>
#include <map>

int main() {<!-- -->
    std::map<int, std::string> myMap;
    
    myMap.emplace(1, "apple");
    myMap.emplace(2, "banana");
    myMap.emplace(3, "orange");
    
    std::cout << "Size of map: " << myMap.size() << std::endl;

    return 0;
}

11. emplace_hint() Function: Constructs a new key-value pair in place at the hint position at a given position.

#include <iostream>
#include <map>

int main() {<!-- -->
    std::map<int, std::string> myMap = {<!-- -->{<!-- -->1, "apple"}, {<!-- -->3, "orange" }};
    
    auto it = myMap.find(3);
    
    myMap.emplace_hint(it, 2, "banana");
    
    std::cout << "Size of map: " << myMap.size() << std::endl;

    return 0;
}