When are header-only libraries acceptable?

Question

Personally, I quite like header-only libraries, but there are claims they cause code bloat due to over-inlining (as well as the other obvious problem of longer compile times

Answer 1

I agree, inline libraries are much easier to consume.

Inline bloat mostly depends on the development platform you are working with - specifically, on the compiler / linker capabilities. I wouldn't expect it to be a major problem wiht VC9 except in a few corner cases.

I've seen some notable change in final size in some places of a large VC6 project, but it's hard to give a specific "acceptable, if...". You probalby need to try with your code in your devenv.

A second problem may be compile times, even when using precompiled header (there are tradeoffs, too).

Third, some constructs are problematic - e.g. static data members shared across translation units - or avoiding having a separate instance in each translation unit.

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I have seen the following mechanism to give the user a choice:

// foo.h
#ifdef MYLIB_USE_INLINE_HEADER
#define MYLIB_INLINE inline
#else 
#define MYLIB_INLINE 
#endif

void Foo();  // a gazillion of declarations

#ifdef MYLIB_USE_INLINE_HEADER
#include "foo.cpp"
#endif

// foo.cpp
#include "foo.h"
MYLIB_INLINE void Foo() { ... }