How do I use a third-party DLL file in Visual Studio C++?

Question

I understand that I need to use LoadLibrary(). But what other steps do I need to take in order to use a third-party DLL file?

I simply jumped into C++ and this is the only part that I do not get (as a Java programmer). I am just looking into how I can use a Qt Library and tesseract-ocr, yet the process makes no sense to me and is so difficult to google.

How do I tell the compiler of the functions that I am using? Should there be an include file from the third-party vendor?

Answer 1

As everyone else says, LoadLibrary is the hard way to do it, and is hardly ever necessary.

The DLL should have come with a .lib file for linking, and one or more header files to #include into your sources. The header files will define the classes and function prototypes that you can use from the DLL. You will need this even if you use LoadLibrary.

To link with the library, you might have to add the .lib file to the project configuration under Linker/Input/Additional Dependencies.

Answer 2

To incorporate third-party DLLs into my VS 2008 C++ project I did the following (you should be able to translate into 2010, 2012 etc.)...

I put the header files in my solution with my other header files, made changes to my code to call the DLLs' functions (otherwise why would we do all this?). :^) Then I changed the build to link the LIB code into my EXE, to copy the DLLs into place, and to clean them up when I did a 'clean' - I explain these changes below.

Suppose you have 2 third-party DLLs, A.DLL and B.DLL, and you have a stub LIB file for each (A.LIB and B.LIB) and header files (A.H and B.H).

• Create a "lib" directory under your solution directory, e.g. using Windows Explorer.
• Copy your third-party .LIB and .DLL files into this directory

(You'll have to make the next set of changes once for each source build target that you use (Debug, Release).)

1. Make your EXE dependent on the LIB files

   • Go to Configuration Properties -> Linker -> Input -> Additional Dependencies, and list your .LIB files there one at a time, separated by spaces: A.LIB B.LIB
   • Go to Configuration Properties -> General -> Additional Library Directories, and add your "lib" directory to any you have there already. Entries are separated by semicolons. For example, if you already had $(SolutionDir)fodder there, you change it to $(SolutionDir)fodder;$(SolutionDir)lib to add "lib".

2. Force the DLLs to get copied to the output directory

   • Go to Configuration Properties -> Build Events -> Post-Build Event
   • Put the following in for Command Line (for the switch meanings, see "XCOPY /?" in a DOS window):

   XCOPY "$(SolutionDir)"\lib\*.DLL "$(TargetDir)" /D /K /Y

   • You can put something like this for Description:

   Copy DLLs to Target Directory

   • Excluded From Build should be No. Click OK.

3. Tell VS to clean up the DLLs when it cleans up an output folder:

   • Go to Configuration Properties -> General -> Extensions to Delete on Clean, and click on "..."; add *.dll to the end of the list and click OK.