What is the correct way to embed a resource in a reusable MFC class?

Question

I am writing a C++ (MFC in particular) class which uses an external .gif image file and produces another image file after some processing. It would be nice if the initial image could be embedded in the code somehow. I have read in MSDN about using multiple .rc files and the whole thing seems quite complicated.

I would like to know from people who have gone through this before how to handle this problem.

EDIT : Sorry I was not clear. The class I am writing should be standalone, so I could use it again. If I put the image in a resource file, then the class will not compile if used in a fresh project.

Answer 1

You cannot embedd MFC resources inside a class or similar C++ container. They can only be embedded in DLL or EXE files - in a separate section of the produced binary. Since you want your class to be reusable, you must put it in a DLL. Hence, you must tag your class with the AFX_EXT_CLASS keyword.

There are two solutions.

Solution #1:

• Create an MFC DLL project (MFC Extension DLL). Call it MyLibrary or whatever.
• Put all your standalone classes in this DLL.
• Embed all necessary resources.
• Let your classes load resources from the HINSTANCE of your DLL as described below.

There are several ways to retrieve the HINSTANCE of your DLL. If you ask me, the best solution is to grab it in DllMain. This is done automatically if you choose the MFC Extension DLL configuration:

static AFX_EXTENSION_MODULE MyLibDLL = { NULL, NULL }; // Make this variable global!
// Then access the hInstance as follows:
LoadResource(MyLibDLL.hModule, ...)

Solution #2:

Store your resource as a byte buffer. Or better, convert it to Base64 and store it as an ASCII string. But remember not to blow the stack! Keep your resources small or increase the stack size in your project settings. Example:

const char *encodedResource = "SGVsbG8gd29ybGQh";
char *data = decode(encodedResource);
foo(data);

Answer 2

In the solution explorer go to resource view, Right click and click Add Resource then click Import and add the gif file. Now you can use your Resource ID to access the gif file in your code.

Answer 3

Just adding the file to a resource doesn't embed the file in the actual resource file it just links to the file. If you open your .rc file you'll see it says something like:

IDB_GIF_MYIMAGE     GIF     "artwork\\mygif.gif"

During the compilation face the resource will be included in the EXE, which you reference using the resource id IDB_GIF_MYIMAGE. You can reference the same file in other projects without having to duplicate the file.

Answer 4

To embed an image (or any other type of binary data) into your class without using resource files, use the bin2c utility, for example you can download it from here: http://www.opensource.apple.com/source/libpcap/libpcap-16/libpcap/msdos/bin2c.c . Running this on a file will produce what is basically a static array with the bytes of the file as members of that array. Stuff this array into a .h file (or put it in the header of your class, or make it a static member...) and then you will have that file available in-memory without having to use LoadResource() and its brethren.

If you want to use this with CImage::Load(), you will have to write your own class that derives from IStream, and implement a few of the methods in a way so that they 'read' from memory. I don't know of any ways to let CImage decode an image from an in-memory representation of a gif file.

Answer 5

I think the best solution is just to document that to use the class you must also import to your project a certain .gif file and give it a certain expected identifier (e.g. IDB_MYCLASS_MYGIF). You can then use the preprocessor to detect if the resource has been correctly added, e.g.:

#ifndef IDB_MYCLASS_MYGIF
    #error Make sure you import mygif.gif to the project.  See docs for more info.
#endif

This will prevent the class compiling until the user imports the image properly. Alternatively you could just use #ifdefs to fall back to code which does not use the default image if it is not provided.

Answer 6

Have a look at the CRuntimeDialog class presented in http://www.codeproject.com/Articles/5371/ToDoList-6-5-4-Feature-Release-An-effective-and-fl . It provides a way to create a dialog from the string that makes up the resource definition.