const auto&x:range

`const auto& x` is a syntax in C++ used to define a constant reference to an element in a range. The `range` here can be any iterable container such as an array, vector, or list.

By using `const auto& x`, you are declaring a constant reference (`const`) named `x` that will refer to each element in the `range`. The `auto` keyword allows the compiler to deduce the type of the elements in the range automatically.

Here's an example to illustrate its usage:

#include <iostream>
#include <vector>

int main() {
    std::vector<int> numbers = {1, 2, 3, 4, 5};

    for (const auto& x : numbers) {
        std::cout << x << " ";
    }

    return 0;
}

In the above code, the `numbers` vector is iterated using a range-based for loop. The `const auto& x` declares a constant reference `x` that will refer to each element in `numbers`. The `const` qualifier ensures that the elements cannot be modified within the loop body.