When programming using the Windows API, I've always made the HINSTANCE from WinMain a global variable immediately. If I want to make an OK button, I'd do it like so (given global HINSTANCE g_hInstance):
return CreateWindow("BUTTON", "OK", WS_TABSTOP|WS_VISIBLE|WS_CHILD|BS_DEFPUSHBUTTON, 10, 10, 100, 30, exampleParentWindow, EXAMPLECHILDID, g_hInstance, NULL);
but lately I've been seeing the instance handle determined without having to be passed as a parameter or clogging up the global namespace, using a call to GetModuleHandle(NULL)*. So, the example above would look like this:
return CreateWindow("BUTTON", "OK", WS_TABSTOP|WS_VISIBLE|WS_CHILD|BS_DEFPUSHBUTTON, 10, 10, 100, 30, exampleParentWindow, EXAMPLECHILDID, GetModuleHandle(NULL), NULL);
*If your compiler supports it, you can write GetModuleHandle(nullptr) and the statement will have the same result.
What's the advantage (if any) of calling GetModuleHandle(NULL) over explicitly specifying the instance handle?
Fine Print: I know this has an answer, but it has not been phrased as its own question on StackOverflow.
In an EXE, it does not make any difference. hInstance from WinMain() and GetModuleHandle(NULL) both refer to the same HINSTANCE (the module of the .exe file). But it does make a difference if you are creating windows inside of a DLL instead, since you have to use the DLL's hInstance but GetModuleHandle(NULL) will still return the HINSTANCE of the EXE that loaded the DLL.
HMODULE WINAPI GetModuleHandle( _In_opt_ LPCTSTR lpModuleName );
Give the module handle of the module name passed.If you are passing NULL, the you get the module handle of the EXE which is currently running.
If you specifically name the module name, you get the module handle of that dll which is mapped to the process address space.
The use is that when you are trying to call a function exported by the dll, or trying to use a dialog template in side that dll.At that time if you use the HMODULE returned form GetMoudleHandle(NULL) your code wont work.
Just to add my two-cents to these answers. In case you need to get the module handle from within a DLL (and don't want to, or can't save it in a global variable from the DllMain call) you can use this function to get it instead:
HMODULE getThisModuleHandle()
{
//Returns module handle where this function is running in: EXE or DLL
HMODULE hModule = NULL;
::GetModuleHandleEx(GET_MODULE_HANDLE_EX_FLAG_FROM_ADDRESS |
GET_MODULE_HANDLE_EX_FLAG_UNCHANGED_REFCOUNT,
(LPCTSTR)getThisModuleHandle, &hModule);
return hModule;
}
One potential gain you get from using GetModuleHandle(NULL) over directly using the WinMain HINSTANCE comes more from architecture. If you want to provide a platform-independent system that runs on linux/windows/whatever you can have a layer that does platform-dependent translations. If that's the case you don't want platform dependent objects such as HINSTANCE showing up in the main application code. So, to circumvent that platform-dependence I put GetModuleHandle(NULL) in the constructor of the platform-dependent class which has the same effect that direct use of the WinMain HINSTANCE does but that abstracts that specific functionality out of the main codebase itself.
Related
I want to a create .dll, which shows some dialog.
In my .dll I have this code:
HWND hDlg = CreateDialogParam(NULL, MAKEINTRESOURCE(IDD_RANKING_DIALOG), NULL, msgProc, NULL);
if (!hDlg) {
ShowError(GetLastErrorAsString().c_str());
return false;
}
ShowError calls Message box and GetLastErrorAsString() just calls standard GetLastError and converts to string.
I have this output:
The specified resource type cannot be found in the image file.
Then I have a standard win32 Window application and there I call method, which calls the mentioned code.
DialogTest test;
test.showDialog(); // calls functionality from .dll
What I am doing wrong? Do I need to link resource files to .dll?
I am using Visual studio 2010 and dialog is specified in my resource file (.rc).
The error code and message are accurate: The resource cannot be found where you instructed the system to go looking for it: The executable image that was used to start the process, not your DLL. The behavior is documented (see CreateDialogParam):
hInstance [in, optional]
Type: HINSTANCE
A handle to the module which contains the dialog box template. If this parameter is NULL, then the current executable is used.
Since you have the dialog template stored inside your DLL, you will have to pass the HINSTANCE that identifies your DLL. There are a number of ways to get the correct value, but passing NULL or GetModuleHandle(NULL) won't work. Both of these return the module handle to the executable image that started the process (not your DLL).
The easy solution: Pick the hInstance passed into your DllMain and store it in a global variable for later use.
HINSTANCE g_hInst = NULL;
BOOL WINAPI DllMain( HINSTANCE hinstDLL, DWORD fdwReason, LPVOID lpvReserved ) {
switch ( fdwReason ) {
case DLL_PROCESS_ATTACH:
g_hInst = hinstDLL;
break;
default:
break;
}
return TRUE;
}
The robust solution: This solution can be used anywhere, in a DLL, an EXE, or a static LIB. Only down-side: It relies on undocumented features of Microsoft's linker. Don't worry, though, it won't fail silently.
EXTERN_C IMAGE_DOS_HEADER __ImageBase;
#define HINST_THISMODULE ((HINSTANCE)&__ImageBase)
HINST_THISMODULE will always hold the correct value, regardless of where it is used.1)
The same can be achieved using official interfaces (GetModuleHandleEx) as well. The following solution can be used from an EXE, a DLL, or a static LIB as well, so long as you make sure to compile and link the function into the respective module:
HMODULE GetCurrentModuleHandle() {
HMODULE hModule = NULL;
GetModuleHandleEx( GET_MODULE_HANDLE_EX_FLAG_FROM_ADDRESS,
(LPCTSTR)GetCurrentModuleHandle,
&hModule );
return hModule;
}
This returns an HMODULE, not an HINSTANCE. This is not an issue, though, since they are the same thing2).
1) From Accessing the current module’s HINSTANCE from a static library
2) What is the difference between HINSTANCE and HMODULE?
You specified NULL as the first parameter to CreateDialogParam. If you want to load the dialog resource file from the Win32 Window Application, you should use this instead:
HWND hDlg = CreateDialogParam(GetModuleHandle(NULL), MAKEINTRESOURCE(IDD_RANKING_DIALOG), NULL, msgProc, NULL);
However, if you want to load it from the DLL itself, you should replace the first parameter with the HINSTANCE parameter from the DllMain entry point function of the DLL.
I have this MFC app that loads strings from string resource using CString::LoadString function. Each of the apps dialogue box's classes and its associated resources are contained in an MFC extension DLL.
CString::LoadString loads strings from the main module's(.exe) resource's string resource successfully but fails to load string from the DLL's resource's string resource.
In each case I get my instance handle for load string from CWinApp object by calling :
CWinApp *WinApp = AfxGetApp(),
and of course the instance's handle is WinApp->m_hInstance which I use as the first argument in my call to CString::LoadString function.
What could be the cause and what could be the solution.
Are you passing the EXE's HINSTANCE to load a string from the extension library? Sounds like it.
With MFC, if you have extension libraries and you make sure your string identifiers are unique, you just need to call the CString::LoadString(UINT nID) version. Because extension libraries create CDynLinkLibrary structures that go into a global linked list, the LoadString(UINT) function will search through all your MFC libraries until it finds the HINSTANCE that contains that string, and then it will load from there. If you insist on using the LoadString() function with an HINSTANCE argument, be sure to use the HINSTANCE of the extension DLL, and not the extension of the EXE when you want to load a string or dialog from the DLL.
For small projects, like less than a dozen DLLs, you probably manage to just use unique IDs for everything. When you get into the insanely large projects like 100+ DLLs, then you have to use other techniques like specially named functions in your DLL that call AfxSetResourceHandle() or know to always use the HINSTANCE of the DLL. That's another topic.
Just to complement Joe Willcoxson's answer, be sure to check that every MFC Extension DLL you are using,needs a special initialization code, similar to the example below:
#include "stdafx.h"
#include <afxdllx.h>
static AFX_EXTENSION_MODULE MyExtDLL = { NULL, NULL };
extern "C" int APIENTRY
DllMain(HINSTANCE hInstance, DWORD dwReason, LPVOID lpReserved)
{
if (dwReason == DLL_PROCESS_ATTACH)
{
TRACE0("MyExt.DLL Initializing!\n");
// Extension DLL one-time initialization
if (!AfxInitExtensionModule(MyExtDLL, hInstance))
return 0;
new CDynLinkLibrary(MyExtDLL);
}
else if (dwReason == DLL_PROCESS_DETACH)
{
TRACE0("MyExt.DLL Terminating!\n");
// Terminate the library before destructors are called
AfxTermExtensionModule(MyExtDLL);
}
return 1; // ok
}
This code just takes care of CDynLinkLibrary creation, the key to share resources between MFC modules.
If all this are setup correctly -- and without ID clashes -- then it's simply a mather of calling:
CString str;
str.LoadString(IDS_MY_STRING_ID);
anywhere in your code, no matter where the string actually persists.
A good start point on resource IDs and numbering can be found in this link.
I have a function that is located in a different cpp file than WinMain is, and when called will create a window. However the windows class for that new window was declared and registered inside of WinMain. Now, when I create the window in the separate function, I am forced to redeclare the 2nd and 3rd variable of the CreateWindowEx function:
_In_opt_ LPCTSTR lpClassName,
_In_opt_ LPCTSTR lpWindowName,
because the location of the function that is actually doing the creating doesn't have access to those variables that were created in WinMain. Now... my doubt is, is my program actually using the window class I registered in WinMain? For all I know, redeclaring these variables, and creating the function far away from the WinMain function could have caused the compiler to adopt some other standard window class. So, is my program actually using the window class I registered in WinMain?
Edited: giving slightly more code
//+++++++++++++++++++++++++++++++++++++cpp file with win main:
INT WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow)
{
...//previous code made the main window of the program and so forth
const char g_szClassName2[] = "windowClass2";//name of the window class
WNDCLASSEX winClass2;
HWND invisHWnd;
winClass2.cbSize = sizeof(WNDCLASSEX);
winClass2.style = 0;
winClass2.lpfnWndProc = WndProcedure2;
winClass2.cbClsExtra = 0;
winClass2.cbWndExtra = 0;
winClass2.hInstance = hInstance;
winClass2.hIcon = LoadIcon(NULL, IDI_APPLICATION);
winClass2.hCursor = LoadCursor(NULL, IDC_ARROW);
winClass2.hbrBackground = (HBRUSH)(COLOR_WINDOW+1);
winClass2.lpszMenuName = NULL;
winClass2.lpszClassName = g_szClassName2;
winClass2.hIconSm = LoadIcon(NULL, IDI_APPLICATION);
RegisterClassEx(&winClass2);
... //other code irrelevant to issue
}
//+++++++++++++++++++++++++++++++++++++other cpp file:
void otherCppFile::creatingWindow()
{
const char g_szClassName2[] = "windowClass2";//name of the window class
const char WndName2[] = "winClass2";//name of the window class
HWND windowHandle = CreateWindowEx(0,
g_szClassName2,
WndName2,
WS_OVERLAPPEDWINDOW,
0,
0,
800,
500,
NULL,
NULL,
NULL,
NULL);
}
Is this last function using the window class registered in WinMain even though it is not located in WinMain and some of the variables are redeclared in the function?
The whole point of registering a window class is so that it is unique and you can use it whenever you need to. When you register a window class using the RegisterClass[Ex] function, you have two options for identifying it:
The window class name (a string) that you passed into the function.
An ATOM (an integer value) that is returned by the function.
In 99.9% of cases, you'll forget about the ATOM and just use the string. You need the string to create the window class, so you might as well just keep up with the string and use it to create windows of that class.
Technically, what you're doing is okay, then. You have multiple variables, but they all contain exactly the same string value, so everything is working properly. But that's a maintenance nightmare, and also violates the DRY (don't repeat yourself) rule. What if you wanted to change the name of your window class? You'd have to find every file that declared it and make sure that you updated it. Then you'd have to wait forever for your entire project to recompile, because you've touched large numbers of files. No good.
Instead, you want to declare this variable at global scope, making it available in every file of your project that creates windows. Put it in a common header file that you're going to include in all of your code files, or make a separate Window.h header or something of that nature and include it where appropriate.
Do make sure that this window class name is declared as a constant. There is no reason it should ever be modified during the execution of your application.
If you're struggling with declaring and/or using global constants in C++, you might check the answers to this question or do a search of your own on Stack Overflow. This part is not unique to Windows programming; it is just basic C++.
Of course, you'll often hear that global variables are bad. That's true enough. The code that you're writing isn't really C++. It is C. The Windows API is a C API, so it naturally leads you into writing C code in C++. If you want to really write C++ code, you'll need to find (or write your own) C++ wrapper around the C-based Windows API. For example, you'd write a class that represents windows, and that class would contain a static class-level constant that holds the name of your window class. The class would be responsible for creating all new windows, so you wouldn't need to scatter that information or those implementation details across multiple code files.
As far as the window name, this is completely different. Windows doesn't use it to identify your window. In fact, it is a completely optional parameter for the CreateWindow[Ex] function. You don't even have to give the window a name. This is actually the name that appears in the window's title bar. It is used by the user to identify the window. As such, it often needs to be localized and should be stored in your project's resources file, not in the source code. Use the LoadString function to load strings from your project's resource file. You can do this dynamically whenever you create a window: just load the string you want to use as the caption for that particular window.
I'm developing a plugin (a dll that is loaded by other app). There's a special function that should return HWND so that app can show a dialog. The problem is that I don't know from where I can get the hInstance (it's not passed as a parameter to the function).
Looks like I'm not understanding something, I'm new to all this stuff, so forgive if the question is silly.
UPD: tried to get hInstance with getModuleHandle:
void* createLoginDialog() {
HINSTANCE hIns = (HINSTANCE) GetModuleHandle(L"comapping");
HWND hWnd = CreateWindow(L"Popup",
L"Enter login",
WS_POPUP,
20,
20,
20,
20,
NULL,
NULL,
hIns,
NULL);
return hWnd;
}
I'm still getting Access Violation.
Several ways to get it:
The first argument passed to your DllMain() entrypoint, cast to HINSTANCE
GetModuleHandle() using your DLL name, cast to HINSTANCE
VirtualQuery(), passing the address of your function. Cast the returned MEMORY_BASIC_INFORMATION.BaseAddress to HINSTANCE. Works on both 32-bit and 64-bit versions of Windows.
According to the documentation of CreateWindow, the hInstance argument is optional. This means that it's valid to pass NULL here. However, as Simon Richter points out in the comments to this answer, the argument may only be NULL the window class is registered globally.
If you're writing a DLL, you may just as well define a DllMain entry point function yourself. This function is called by Windows, passing the handle of your function as the first arugment. You can memorize this handle somewhere to reuse it when creating your window.
I am now working on a some sort of a game engine and I had an idea to put everything engine-related into a static library and then link it to my actual problem.
Right now I achieved it and actually link that library and every functions seem to work fine, except those, which are windows-related.
I have a chunk of code in my library that looks like this:
hWnd = CreateWindow(className, "Name", WS_OVERLAPPED | WS_CAPTION | WS_EX_TOPMOST,
0, 0,
800, 600,
NULL, NULL, GetModuleHandle(NULL), this);
if (hWnd) {
ShowWindow(hWnd, SW_NORMAL);
UpdateWindow(hWnd);
} else {
MessageBox(NULL, "Internal program error", "Error", MB_OK | MB_ICONERROR);
return;
}
When this code was not in the library, but in the actual project, it worked fine, created the window and everything was ok. Right now (when I'm linking to my library that contains this code) CreateWindow(...) call returns NULL and GetLastError() returns "Operation succesfully completed" (wtf?).
Could anybody help me with this? Is it possible to create a window and display it using a static library call and why could my code fail?
Thank you.
Ah, maybe you've run into this problem described in an MSDN blog:
If you're writing a static library, you may have need to access the HINSTANCE of the module that you have been linked into. You could require that the module that links you in pass the HINSTANCE to a special initialization function, but odds are that people will forget to do this.
If you are using a Microsoft linker, you can take advantage of a pseudovariable which the linker provides.
EXTERN_C IMAGE_DOS_HEADER __ImageBase;
#define HINST_THISCOMPONENT ((HINSTANCE)&__ImageBase)
The pseudovariable __ImageBase represents the DOS header of the module, which happens to be what a Win32 module begins with. In other words, it's the base address of the module. And the module base address is the same as its HINSTANCE.
So there's your HINSTANCE.
So, instead of passing GetModuleHandle(NULL) to CreateWindow, try ((HINSTANCE)&__ImageBase) (make sure it is declared as shown in the blog first).
Edit:
From the comments in that blog entry, one mentions the use of GetModuleHandleEx(), perhaps this is a more Compiler/Linker-agnostic approach.