Compile header-only template library into a shared library?

Question

We are in the process of designing a new C++ library and decided to go with a template-based approach along with some specific partial template specialisations for corner ca

Answer 1

If it is really templates-only, then there is no shared library. See various Boost projects for concrete examples. Only when you have non-template code will you have a library. A concrete example is eg Boost Date_Time and date formatting and parsing; you can use the library with or without that feature and hence with or without linking.

Not having a shared library is nice in the sense of having fewer dependencies. The downside is that your binaries may get a little bigger and that you have somewhat higher compile-time costs. But storage is fairly cheap (unless you work in embedded systems are other special circumstances) and compiling is usually a fixed one-time cost.

Answer 2

Although there isn't a standard way to do it, it is usually possible with implementation specific techniques. I did it a long time ago with Borland's C++ Builder. The idea is to declare your templates to be exported from the shared library where they need to reside and import them where they are used. The way I did it was along these lines:

// A.h
#ifdef GENERATE
#  define DECL __declspec(dllexport)
#else
#  define DECL __declspec(dllimport)
#endif

template <typename T> class DECL C {
};

// A.cpp
#define GENERATE
#include "A.h"

template class DECL A<int>;

Beware that I don't have access to the original code, so it may contain mistakes. This blog entry describes a very similar approach.

From your wording I suspect you're not on Windows, so you'll have to find out if and how this approach can be adopted with your compiler. I hope this is enough to put you in the right direction.

Answer 3

C++ Shared Library with Templates: Undefined symbols error Some answers there cover this topic. To sum up short: it is possible if you force to instantiate templates in shared library code explicitly. It will require explicit specification for all used types for all used templates on shared lib side, though.

Answer 4

Yes. What you can do is explicitly instantiate the templates in CPP files using the compiler's explicit template instantiation syntax. Here is how to use explicit instantiation in VC++: http://msdn.microsoft.com/en-us/library/by56e477(v=VS.100).aspx. G++ has a similar feature: http://gcc.gnu.org/onlinedocs/gcc/Template-Instantiation.html#Template-Instantiation.

Note that C++11 introduced a standard syntax for explicit instantiation, described in [14.7.2] Explicit instantiation of the FDIS:

The syntax for explicit instantiation is:

explicit-instantiation:

extern_opt template declaration