One of my colleagues was frustrated trying to get some code to build. “Is there something strange about linking variadic functions? Because I keep getting an unresolved external error for the function, but if I move the function definition to the declaration point, then everything works fine.”

// blahblah.h
... other declarations ...
void LogWidget(Widget* widget, const char* format, ...);
...
// widgetstuff.cpp
...
#include "blahblah.h"
...
// some code that calls LogWidget
void foo(Widget* widget)
{
 LogWidget(widget, "starting foo");
 ...
}
// and then near the end of the file
void LogWidget(Widget* widget, const char* format, ...)
{
    ... implementation ...
}
...

“With the above code, the linker complains that Log­Widget cannot be found. But if I move the implementation of Log­Widget to the top of the file, then everything builds fine.”

// widgetstuff.cpp
...
#include "blahblah.h"
...
// move the code up here
void LogWidget(Widget* widget, const char* format, ...)
{
    ... implementation ...
}
// some code that calls LogWidget
void foo(Widget* widget)
{
 LogWidget(widget, "starting foo");
 ...
}
...

“I tried putting an explicit calling convention in the declaration, I tried using extern "C", nothing seems to help.”

We looked at the resulting object file and observed that in the case where the error occurred, there was an external reference to Log­Widget but no definition. I asked, “Is the definition of the function #ifdef‘d out by mistake? You can use this technique to find out.”

That was indeed the problem. The definition of the function was inside some sort of #ifdef that prevented it from being compiled.

Sometimes, the reason a function cannot be found is that it doesn’t exist in the first place.