Change input method on windows using c++ and windows API - c++

Question

I want to write some code in C++ in combination with windows API in order to change the input method on windows (Keyboard input language).
Here is my first try:
#include <iostream>
#include <windows.h>
int main() {
int arr_size = GetKeyboardLayoutList(0, NULL);
HKL hkl_array[arr_size]; // The array to hold input locale identifies found in the system(HKL)
// Initialize the array to all zeros
for (int i = 0; i <= arr_size - 1; i++) {
hkl_array[i] = nullptr;
}
int lang_found = GetKeyboardLayoutList(arr_size, hkl_array); // Get all the HKLs in the array
for (int i = 0; i <= lang_found - 1; ++i) {
// Print all the HKLs found
std::cout << i + 1 << ". " << hkl_array[i] << std::endl;
}
std::cout << "Function return: " << ActivateKeyboardLayout(hkl_array[0], KLF_SETFORPROCESS);
return 0;
}
When running the above code with English as the current language I get:
1. 0x4080408
2. 0x4090409
Function return: 0x4090409
(0x4080408 is Greek and 0x4090409 is English)
According to the documentation for ActivateKeyboardLayout, the return value is of type HKL and If the function succeeds, the return value is the previous input locale identifier. Otherwise, it is zero. In my case, the function clearly runs with no errors since it returns the input locale identifier for English witch was the language before the hypothetical change. The language although does not change. I get similar results if I start from greek and try to change to English.
I have tried different values in the flag parameter with no luck. It is always the same thing. I have also tried using the LoadKeyboardLayoutA as follows:
#include <iostream>
#include <windows.h>
int main(){
LPCSTR language_select = "00000408"; // Code for greek
std::cout << LoadKeyboardLayoutA(language_select, KLF_ACTIVATE);
return 0;
}
When running the above code with English as the current language I get:
0x4080408
According to the documentation for LoadKeyboardLayoutA, If the function succeeds, the return type is HKL and the return value is the input locale identifier corresponding to the name specified in the function's parameter (language_select). So this seems to run ok as well but no luck with language change. I also tried with language_select = "00000809" (UK English) and the result I got was Uk English was just added to the list of languages as I did not have it installed.
Finally, I tried calling the ActivateKeyboardLayout with LoadKeyboardLayoutA ad the HKL parameter as follows
#include <iostream>
#include <windows.h>
int main() {
LPCSTR language_select = "00000408"; // Code for greek
std::cout << ActivateKeyboardLayout(LoadKeyboardLayoutA(language_select, KLF_ACTIVATE), KLF_SETFORPROCESS);
return 0;
}
When running the above code with English as the current language I get:
0x4080408
Which again seems normal but there is no chance in the language. I am running this on Windows 10 20H2 OS Build 19042.685 and I am using CLion as an IDE (I don't think it matters but just in case anyone needs the info). What am I doing wrong?

Answer 1

The very first sentence of the ActivateKeyboardLayout function documentation states: "Sets the input locale identifier (formerly called the keyboard layout handle) for the calling thread or the current process.". Since you never read input in your program, you don't observe its effect. It does not change the keyboard layout of the system.
On Windows, each thread has its own keyboard layout. To change the keyboard layout for a specific window, you need to send the WM_INPUTLANGCHANGEREQUEST message to it:
SendMessageW(<windowHandle>, WM_INPUTLANGCHANGEREQUEST, 0, LoadKeyboardLayoutW(language_select, KLF_ACTIVATE));