Unresolved external UrlCombineW

Question

I have a file which looks something like this:

#include <Shlwapi.h>
...
void SomeFunction()
{
UrlCombineW(...)
}

This compiled just fine until I installed another Delphi component in C++ Builder IDE but now reports unresolved external for UrlCombineW. The above call was fine before installing this component.

It seems that the component is overwriting this in some way so I need to explicitly tell the compiler where to look for UrlCombineW. This is a function from Shlwapi.dll.

Compiler does not complain, but how do I explicitly tell the linker where to look for this function and avoid unresolved external error?

Answer 1

Expanding my comment to an answer.

You need to link to Shlwapi.lib in order for the linker to find the functions. (This explanation glosses over a few things, but a .lib, a library file, can be either a static or import library. A static library contains the functions themselves - it's basically a collection of .obj files bundled together; an import library says that functions X, Y and Z are found in a specific DLL.) Either way, if you link the .lib in you will get the functions that you need.

There are a couple of ways to do tell the linker to link in the file:

• Use #pragma comment(lib, "Filename.lib") in a .cpp file somewhere. For your case, this is #pragma comment(lib, "Shlwapi.lib").
• Add it to the project options, which in turn adds it to the linker command line. In C++ Builder you do this by actually adding the .lib file to the project, ie drag and drop it onto the project in the Project Manager, or use File > Add To Project.

Which you prefer is up to you. I tend to link to localized things locally - so in my code, there's only one unit which uses Shlwapi.h and the fact it does so is an implementation detail hidden from the outside, it's not shown in the interface. Therefore, in that file, I link using #pragma comment at the point I include the header. On the other hand, if you have something used far more widely - to pick the widest example, kernel32.lib - I would add that the project itself. (Note this is an example, you don't actually need to explicitly link to kernel32, that will be done for you!)