Determining the position of the first occurrence of a key in a sorted array

Given a sorted array data in ascending order. Implement a binary search algorithm that returns the index of the first occurrence of the specified value target. If the element is not found, the function should return -1.

int first(const std::vector<int> arr, int key) {
    // implement
}

Examples of the function's operation:

• data = {1, 2, 2, 2, 3}, target = 2 → result 1
• data = {1, 3, 5, 7}, target = 4 → result -1