Dynamic and Static Libraries in C++

Question

In my quest to learn C++, I have come across dynamic and static libraries.

I generally get the gist of them: compiled code to include into other programs.

Howeve

Answer 1

Is writing [libraries] any different than a normal C++ program, minus the main() function?

That depends on your definition of "different." From the language's point of view, you write a file or collection of files, don't put in a main() and you tell the compiler to generate a library instead of an executable.

However, designing libraries is much harder because you have no control over the code that calls you. Libraries need to be more robust against failure than normal code. You can't necessarily delete pointers somebody passes to your function. You can't tell what macros will mess with your code. You also can't accidentally pollute the global namespace (eg., don't put using namespace std at the beginning of your header files).

How does the compiled program get to be a library? It's obviously not an executable, so how do I turn, say 'test.cpp' into 'test.dll'?

That depends on the compiler. In Visual C++ this is a project config setting. In gcc (going from memory) it's something like gcc -c foo.c -shared.

Once I get it to its format, how do I include it in another program?

That depends on your compiler and linker. You make sure the header files are available via a project setting or environment variable, and you make sure the binaries are available via a different project setting or compiler variable.

Is there a standard place to put them, so that whatever compilers/linkers need them can find them easily?

That depends on the operating system. In UNIX you're going to put things in places like /usr/lib, /usr/local/lib. On Windows people used to put DLLs in places like C:\WINDOWS but that's no longer allowed. Instead you put it in your program directory.

What is the difference (technically and practically) between a dynamic and static library?

Static libraries are the easier, original model. At compile time the linker puts all the functions from the library into your executable. You can ship the executable without the library, because the library is baked in.

Dynamic libraries (also called shared libraries) involve the compiler putting enough information in the executable that at runtime the linker will be able to find the correct libraries and call the methods in there. The libraries are shared across the whole system among the programs that use them. Using dynamic linking (dlsym(), et. al.) adds a few details to the picture.

How would I use third party libraries in my code (I'm staring at .dylib and .a files for the MySql C++ Connector)

That's going to depend on your platform, and unfortunately I can't tell you much about .dylib files. .a files are static libraries, and you simply need to add them to your final call to gcc (gcc main.c foo.a -o main if you know where foo.a is, or gcc main.c -lfoo -o main if the system knows where foo.a, foo.la, or foo.so are). Generally you make sure the compiler can find the library and leave the linker to do the rest.