Reverse an vector

Here's a simple function in C++ that reverses a vector:

```cpp
#include <iostream>
#include <vector>

// Function to reverse a vector
void reverseVector(std::vector<int>& vec) {
    int left = 0;
    int right = vec.size() - 1;

    while (left < right) {
        // Swap elements at left and right indices
        std::swap(vec[left], vec[right]);
        left++;
        right--;
    }
}

// Function to print a vector
void printVector(const std::vector<int>& vec) {
    for (const auto& element : vec) {
        std::cout << element << " ";
    }
    std::cout << std::endl;
}

int main() {
    // Create a sample vector
    std::vector<int> vec = {1, 2, 3, 4, 5};

    std::cout << "Original vector: ";
    printVector(vec);

    // Reverse the vector
    reverseVector(vec);

    std::cout << "Reversed vector: ";
    printVector(vec);

    return 0;
}
```

This code defines two functions: `reverseVector` and `printVector`. The `reverseVector` function takes a reference to a vector as an argument and reverses its elements in-place. The `printVector` function takes a constant reference to a vector and prints its elements.

In the `main` function, we create a sample vector, print it, reverse it using the `reverseVector` function, and then print the reversed vector.

When you run this code, it will output:

```
Original vector: 1 2 3 4 5
Reversed vector: 5 4 3 2 1
```

This code uses the `std::swap` function to swap elements at the left and right indices of the vector, effectively reversing the vector. The `left` and `right` indices are initialized to the start and end of the vector, respectively, and then incremented/decremented in each iteration of the loop until they meet in the middle.