write a C or C++ library with "template"

Question

Hi,

(1). When using C++ template, is it correct that the compiler (e.g. g++) will not compile the template definition (which can only be in header file not source file) directly, but generate the code based on template definition for each of its instantiations and then compile the generated code for its instantiations?

(2). If I want to write a C++ library which provide template classes and template functions, is it impossible to compile the library into shared file (.so, .a) because their instantiations will not be anywhere in the code of the library but only appear in the user's program? If yes, does it mean that template libraries are just source code files not precompiled files?

How is C++ standard template library (STL) implemented? Is its source code precompiled or compiled together with user's program?

(3). In C,

how to write a library that provide functions acting like template functions in C++? Is overloading a good solution?

If I have to write a procedure into a different function for different types of arguments, is there a good way for code reusing? Is this a good way to do it http://www.vlfeat.org/api/imop_8c_source.html? Any other ways?

Thanks and regards!

Answer 1

When using C++ template, is it correct that the compiler (e.g. g++) will not compile the template definition.

Yes. It's a correct assumption.

A template definition is incomplete code. You need to fill in the template parameters before compiling it.

If I want to write a C++ library which provide template classes and template functions, is it impossible to compile the library into shared file (.so, .a)

No it's not possible. You can only compile individual instantiations of a template.

How is C++ standard template library (STL) implemented? Is its source code precompiled or compiled together with user's program?

A large part of the STL code resides in header files and gets compiled together with your application.

In C, how to write a library that provide functions acting like template functions in C++? Is this a good way to do it http://www.vlfeat.org/api/imop_8c_source.html? Any other ways?

Including the same file multiple times after redefining a macro (as demonstrated in the link you provided) is a good way to do this.

Answer 2

(3). In C, how to write a library that provide functions acting like template functions in C++? Is overloading a good solution?

If I have to write a procedure into a different function for different types of arguments, is there a good way for code reusing? Is this a good way to do it http://www.vlfeat.org/api/imop_8c_source.html? Any other ways?

When I need to write general purpose code I use void * as basic data type. This is good because it allows you to store both a generic pointer and a "primitive" value (like a int). Also recently I had to compile some code using this pattern in a 64 bit environment, and this taught me the importance of the stdint.h data types!

Speaking of acting like template in C, this is not a good idea. This is just my opinion, of course, but I think that the strong point of C is its simplicity, which is the reason why C is less error prone than C++.