C++ flat_set::begin() Function

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The std::flat_set::begin() function in C++, is used to return an iterator pointing to the first element of the flat_set. A flat_set is a sorted container that stores unique elements in a flat structure, typically implemented using a vector.

The begin() function allows traversal of elements in a sorted order. It comes in two forms, the non-const version returning an iterator and a const version returning a const_iterator.

Syntax

Following is the syntax for std::flat_set::begin() function.

 iterator begin() noexcept; or const_iterator begin() const noexcept; 

Parameters

It does not accepts any parameter.

Return Value

This function returns the iterator to the first element.

Example 1

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

 #include <iostream> #include <boost/container/flat_set.hpp> int main() { boost::container::flat_set < int > x = {321,23,121}; auto a = x.begin(); if (a != x.end()) { std::cout << "Result : " << * a << std::endl; } return 0; } 

Output

Output of the above code is as follows −

 Result : 23 

Example 2

Consider the following example, where we are going to define the flat_set with the strings, and then the begin() is used to iterate over the set using the loop.

 #include <iostream> #include <boost/container/flat_set.hpp> int main() { boost::container::flat_set < std::string > a = {"Welcome","Hi","Hello"}; for (auto x = a.begin(); x != a.end(); ++x) { std::cout << * x << " "; } std::cout << std::endl; return 0; } 

Output

Output of the above code is as follows −

 Hello Hi Welcome 

Example 3

In the following example, where we are going to use the begin() along with the cbegin() for comparison.

 #include <iostream> #include <boost/container/flat_set.hpp> int main() { boost::container::flat_set < char > a = {'d','y','b'}; auto x1 = a.begin(); auto x2 = a.cbegin(); if (x1 == x2) { std::cout << "Both points to the same element: " << * x1 << std::endl; } return 0; } 

Output

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

 Both points to the same element: b