C++ multimap::crbegin() Function

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The C++ std::multimap::crbegin() function is used to return a constant reverse iterator to the last element, enabling read-only traversal from the end to the beginning of the container. This function ensures that the elements are accessed in reverse order without allowing modification. It is mainly useful in the secnarios for read-only operations like accessing and inspecting elements in reverse order. The time complexity of this function is constant i.e.O(1).

Syntax

Following is the syntax for std::multimap::crbegin() function.

const_reverse_iterator crbegin() const noexcept;

Parameters

It does not accept any parameter

Return value

This function returns a constant reverse iterator.

Example

Let's look at the following example, where we are going to demonstrate the basic usage of crbegin() function.

#include <iostream>
#include <map>
int main()
{
    std::multimap<int, std::string> a = {{1, "Hello"},{3, "Namaste"}, {2, "Hi"}};
    auto x = a.crbegin();
    std::cout <<"(" << x->first << ", " << x->second << ")\n";
    return 0;
}

Output

Output of the above code is as follows −

(3, Namaste)

Example

Consider the following example, where we are going to use crbegin() to iterate over the multimap in reverse order. It iterates until it reaches crend().

#include <iostream>
#include <map>
int main()
{
    std::multimap<int, std::string> a = {{2, "TP"}, {3, "TutorialsPoint"}, {1, "Tutorix"}};
    for (auto x = a.crbegin(); x != a.crend(); ++x) {
        std::cout << "(" << x->first << ", " << x->second << ")\n";
    }
    return 0;
}

Output

Following is the output of the above code −

(3, TutorialsPoint)
(2, TP)
(1, Tutorix)

Example

In the following example, we are going to find the specific element in reverse order.

#include <iostream>
#include <map>
int main()
{
    std::multimap<int, std::string> a = {{3, "BMW"}, {1, "Cruze"}, {2, "Sail"}};
    for (auto x = a.crbegin(); x != a.crend(); ++x) {
        if (x->first == 1) {
            std::cout << "" << x->second << std::endl;
            break;
        }
    }
    return 0;
}

Output

Let us compile and run the above program, this will produce the following result −

Cruze

Example

Following is the example, where we are going to compare the values in the reverse order.

#include <iostream>
#include <map>
int main()
{
    std::multimap<int, std::string> x = {
        {1, "A"}, {2, "B"}, {3, "C"}
    };
    std::multimap<int, std::string> y = {
        {3, "C"}, {2, "B"}, {1, "A"}
    };
    bool equal = std::equal(x.crbegin(), x.crend(), y.crbegin(), y.crend());
    std::cout << (equal ? "Multimaps are equal." : "Multimaps are not equal.") << '\n';
    return 0;
}

Output

If we run the above code it will generate the following output −

Multimaps are equal.