C++ Utility::swap() Function

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The C++ std::utility::swap() function is used to exchange rhe values of the two variables. It efficiently swaps the contents of the given objects by performing a copy or move operation, depending on whether the types support move semantics. The time complexity of this function is constant O(1).

Syntax

Following is the syntax for std::utility::swap() function.

void swap (T& a, T& b)  noexcept (is_nothrow_move_constructible<T>::value && is_nothrow_move_assignable<T>::value);
or
void swap(T (&a)[N], T (&b)[N])  noexcept (noexcept(swap(*a,*b)));

Parameters

• a, b − It indicates the two objects whose contents are swapped.

Return Value

none

Exceptions

If the construction or assignment of type T throws.

Data races

Both a and b are modified.

Example 1

In the following example, we are going to consider the basic usage of the swap() function.

#include <iostream>
#include <utility>
int main() {
   int a = 1, b = 2;
   std::swap(a, b);
   std::cout << "After swap: a = " << a << ", b = " << b << std::endl;
   return 0;
}

Output

Output of the above code is as follows −

After swap: a = 2, b = 1

Example 2

Consider the following example, where we are going to swap the two arrays.

#include <iostream>
#include <utility>
int main() {
   int x[] = {1,3};
   int y[] = {2,4};
   std::swap(x, y);
   std::cout << "Array 1 after swap: ";
   for (int i: x) std::cout << i << " ";
   std::cout << "\nArray 2 after swap: ";
   for (int i: y) std::cout << i << " ";
   return 0;
}

Output

Following is the output of the above code −

Array 1 after swap: 2 4 
Array 2 after swap: 1 3

Example 3

Let's look at the following example, where we are going to swap the custom objects.

#include <iostream>
#include <utility>
class Point {
   public: int a,
   b;
   Point(int a, int b): a(a),
   b(b) {}
};
int main() {
   Point x(11, 22), y(33, 44);
   std::swap(x, y);
   std::cout << "Result : x(" << x.a << ", " << x.b << "), y(" << y.a << ", " << y.b << ")\n";
   return 0;
}

Output

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

Result : x(33, 44), y(11, 22)