Condensing Declaration and Implementation into an HPP file.

Question

I've read a few of the articles about the need / applicability / practicality of keeping headers around in C++ but I can't seem to find anywhere a solid reason why / when the above should or should not be done. I'm aware that boost uses .hpp files to deliver template functions to end users without the need for an associated .cpp file, and this thought is partially sourced off browsing through that code. It seems like this would be a convenient way to deliver single file modules of say a new Wt or Qt widget (still sticking to the one class per .h convention).

However are there any negative technical implementations for giving somebody a single .hpp file with both the header declaration and implementation assuming you have no problem with them having access to the implementation (say in the context of OSS). Does it for instances have any negative implications from the compiler's / linker's perspective?

Any opinions or perspectives on this would be appreciated.

Answer 1

'm aware that boost uses .hpp files to deliver template functions to end users without the need for an associated .cpp file

Wrong verb: it’s not “without the need”, it’s “without the ability”.

If Boost could, they would separate their libraries into headers and implementation files. In fact, they do so where ever possible.

The reason for a clean separation is simple: compilation time for header-only projects increases tremendously because associated header files have to be read, parsed and compiled every time you recompile the tiniest part of your application.

Implementation files only need to be compiled if you happen to recompile that particular object file.

Large C and/or C++ projects take hours to compile. And these use a clean separation into header and object files. If they would only use header files, I’m betting the compilation time would be measured in days instead of hours.

But for many of Boost’s libraries, the fact is that template definitions may not reside in a separate compilation unit than their declarations so this is simply not possible.

Answer 2

The major negative aspect of .hpp-only libraries is that they cannot refer to a precompiled module. All of the code present in the .hpp and hence all of the code in the library must be added to your application. This increases the size of the binary and makes for redundant binaries on such a system that uses the library more than once.

Answer 3

With templates you have no real choice. In theory, export allows you to separate the interface from the implementation, but only one compiler (Comeau) really supports this¹, and it's being dropped from C++0x.

In any case, trying to put the implementations of non-template functions into headers leads to one obvious problem: the One Definition Rule remains in effect, so if you define the same function in more than one translation unit, you have a problem. The linker will typically give an error saying the same symbol has been defined more than one.

¹Though it's mostly the EDG compiler front-end that really supports it, so other EDG-based compilers, such as Intel's also support export to some degree, though they don't document it, so you can't depend on much with them.