Lars at the OSL

Here’s a C++ issue that’s been stumping me all morning. I’ve got a compiler error in a header file that states that a particular symbol is not declared within the current scope. I’ve been able to figure out is that the compiler is right - I certainly cannot find any way that this symbol is available anywhere that any compiler would be able to find it. A mitigating factor is that the missing symbol is within the declaration of a template.

Grepping across all files in the project, I can find the symbol repeated in many files as a local static constant.

static const char kValueDescription[] = "Writes Null Files";

This same definition appears in many files, each file has its own unique string. So these definitions of my missing symbol are available only after the inclusion of my header file. Just to test, I moved the include of the header from the top of the cpp file to a point below the alleged definition of my missing symbol: the missing symbol error goes away. It just seems plain wrong to require something to be defined prior to inclusion of a header file. Header files should be self contained: it should forward declare classes it can’t know details about or include other headers to get the declarations that it needs.

So here’s what I think is going on: the code is banking on the assumption that the compiler will blindly treat the template as if it were a macro. The original author wanted the compiler to not process the template until it is actually instantiated (coincidentally immediately after the the declaration/definition of the missing symbol). In my case, the compiler is not cooperating and as far as the source file is concerned, compiling the template prematurely.

I know that the GNU C++ compiler has had a bug in the past that where templates were instantiated prematurely. I’m not yet sure if this is an instance of this problem. I’ve seen the premature template instantiation problem manifest as an unlawful symbol redefinition, not a missing symbol. The question is, should a compiler fully check a template prior to instantiation? I would say “yes”, unfortunately, this code says, “no.”

I need to find a solution. Either I have to make the new location of the include permanent, an option I find distasteful, or I need to find a way to forward declare the static const char array in the header file. Unfortunately, forward declarations are for classes, not data items. How do you forward declare data?

So I am forced to be unclean. I’ve moved the header inclusion from the top of the file where it belongs down into the middle of the source file. This makes me ill with foreboding that there will be more trouble in the future from this.

This entry was posted on Monday, July 9th, 2007 at 5:06 pm and is filed under Open Source, Helix Producer, C++. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.