C++ Getting screen coordinates of window client-area from relative coordinates

get_window_client-area_coordinates.cpp

// Window client-area is the area without Title bar,...

#include <Windows.h>

RECT rct;
// Getting upper-left and lower-right corners relative coordinates.
if (!GetClientRect(someHWND, &rct))
{
    std::cout << "[GetClientRect] Error " << GetLastError() << '\n';
    return;
}

// Because client coordinates are relative to the upper-left corner of a window's client area,
// the relative coordinates of the upper-left corner are always (0,0).
POINT top_left;
top_left.x = 0;
top_left.y = 0;

POINT bottom_right;
bottom_right.x = rct.right;
bottom_right.y = rct.bottom;

// Convert 2 corner relative coords to screen coords
if (!ClientToScreen(someHWND, &top_left) || !ClientToScreen(someHWND, &bottom_right))
{
    std::cout << "[ClientToScreen] Error " << GetLastError() << '\n';
    return;
}