Convert HWND to Hex String in C++

Question

What is the best way to convert HWND to Hex String in C++, I mean also with a "0x" prefix?

HWND hWnd = FindWindow(L"Notepad", L"Untitled - Notepad");
MessageBox(nullptr, LPCWSTR(hWnd), L"Hello World!", MB_ICONINFORMATION | MB_OK | MB_DEFBUTTON1);

But I expect this to output 0x00000000 (assuming Notepad windows is not open) , but it always returns an empty string.

I also tried this answer , but I ended up with returning 0000000000000000 .

Anyone can help me on that conversion?

Answer 1

To get a string representation of a hexadecimal number insert the 0x literal followed by a handle into a stringstream:

#include <Windows.h>
#include <sstream>
#include <iostream>

int main(){
    HWND hWnd = FindWindow(L"Notepad", L"Untitled - Notepad");
    std::stringstream ss;
    ss << "0x" << hWnd;
    std::cout << ss.str();
}

If you need to print out the result in a MessageBox use wide stringstream:

#include <Windows.h>
#include <sstream>

int main(){
    HWND hWnd = FindWindow(L"Notepad", L"Untitled - Notepad");
    std::wstringstream wss;
    wss << "0x" << hWnd;
    MessageBox(NULL, wss.str().c_str(), L"Hello World!", MB_ICONINFORMATION | MB_OK | MB_DEFBUTTON1);
}

Answer 2

What you are doing is not conversion. You just cast hWnd to a pointer to string. Almost always it will not point to a valid string, producing an undefined behavior when you try to print it as a string.

To do it properly, you should trait hWnd's bit's as integer and print it to some buffer as hex before showing in the message box:

#include <sstream>
#include <cstdint>
#include <iomanip>

//.....

std::wstringstream ss;
ss << std::hex << L"0x" << std::setw(16) << std::setfill(L'0') << 
    *reinterpret_cast<uint64_t*>(&hWnd) << std::endl;
MessageBox(nullptr, ss.str().c_str(), L"Hello World!",
    MB_ICONINFORMATION | MB_OK | MB_DEFBUTTON1);

Notes:

1) stringstream is a C++-style sprintf . It's str() method returns std::string , so to get a C-style pointer you should call c_str on it.

2) I have no Windows to check what is HWND actually. So please check it's size and use appropriate integer type instead of uint64_t . It's important, as if you use a too wide type, you'll get garbage or even access violation. A better approach is to use an integer type template like one discussed here .

3) Probably, you need std::wstringstream as you are using wide-char version of MessageBox.

4) Decoration. ss << *reinterpret_cast<uint64_t*>(&hWnd) just prints raw hex digits to ss , so to get the right format, you should fine-tune it, setting proper padding and filling character. For example, this will cause all integers to be printed as 16-digit numbers with leading zeros:

ss << std::setw(16) << std::setfill(L'0') ...

where setw and setfill functions are from iomanip header. Also, printing 0x prefix is your job, not stringstream 's. Also take a look at std::showbase .

Answer 3

A somewhat hackish solution that works for integers is to cast the type to (void *) iE a pointer to void type. On current Microsoft compilers, this nicely prints it prefixed with 0x because it is interpreted as "address".

int main(){
    HWND hWnd = FindWindow(L"Notepad", L"Untitled - Notepad");
    std::wstringstream wss;
    wss << (void*)hWnd;
    MessageBox(NULL, wss.str().c_str(), L"Hello World!", MB_ICONINFORMATION | MB_OK | MB_DEFBUTTON1);
}

This trick works out-of-the-box with Microsoft Visual C++ compilers. Gcc and clang may issue a warning which can be suppressed if really necessary (probably -Wint-to-pointer-cast ).