Will g++ link my programs with classes it doesn't use from a library?

Question

I've created a simple static library, contained in a .a file. I might use it in a variety of projects, some of which simply will not need 90% of it. For example, if I want to use neural networks, which are a part of my library, on an AVR microcomputer, I probably wont need a tonne of other stuff, but will that be linked in my code potentially generating a rather large file?

I intend to compile programs like this:

g++ myProg.cpp myLib.a -o prog

Answer 1

The GNU linker will pull in the stuff it needs from the libraries you have specified on an object file by object file basis. Object files are atomic units as far as the GNU linker is concerned. It doesn't split them apart. The linker will bring in an object file if that object file defines one or more unresolved external references. That object file may have external references. The linker will try to resolve these, but if it can't, the linker adds those to the set of references that need to be resolved.

There are a couple of gotchas that can make for a much larger than needed executable. By larger than needed, I mean an executable that contains functions that will never be called, global objects that will never be examined or modified, during the execution of the program. You will have binary code that is unreachable.

One of these gotchas results when an object file contains a large number of functions or global objects. Your program might only need one of these, but your executable gets all of them because object files are atomic units to the linker. Those extra functions will be unreachable because there's no call path from your main to these functions, but they're still in your executable. The only way to ensure that this doesn't happen is to use the "one function per source file" rule. I don't follow that rule myself, but I do understand the logic of it.

Another set of gotchas occur when you use polymorphic classes. A constructor contains auto-generated code as well as the body of the constructor itself. That auto-generated code calls the constructors for parent classes, inserts a pointer to the vtable for the class in the object, and initializes data members per the initializer list. These parent class constructors, the vtable, and the mechanisms to process the initializer list might be external references that the linker needs to resolve. If the parent class constructor is in a larger header file, you've just dragged all that stuff into your executable.

What about the vtable? The GNU compiler picks a key member function as the place to store the vtable. That key function is the first member function in the class that does not have a an inline definition. Even if you don't call that member function, you get the object file that contains it in your executable -- and you get everything that that object file drags in.

Keeping your source files down to a small size once again helps with this "look what the cat dragged in!" problem. It's a good idea to pay special attention to the file that contains that key member function. Keep that source file small, at least in terms of stuff the cat will drag in. I tend to put small, self-contained member functions in that source file. Functions that will inevitably drag in a bunch of other stuff shouldn't go there.

Another issue with the vtable is that it contains pointers to all of the virtual functions for a class. Those pointers need to point to something real. Your executable will contain the object files that define each and every virtual function defined for a class, including the ones you never call. And you're going to get everything that those virtual functions drag in as well.

One solution to this problem is to avoid making big huge classes. They tend to drag in everything. God classes in particular are problematic in this regard. Another solution is to think hard about whether a function really does need to be virtual. Don't just make a function virtual because you think someday someone will need to overload it. That's speculative generality, and with virtual functions, speculative generality comes with a high cost.

Answer 2

You could also try to break your library into independent smaller parts and only link the parts you are really going to need.

Answer 3

G++ will pull in only the object files it needs from your library, but this means that if one symbol from a single object file is used, everything in that object file gets added to your executable.

One source file becomes one object file, so it makes sense to logically group things together only when they are sure to be needed together.

This practice varies by compiler (actually by linker). For example, the Microsoft linker will pick object files apart and only include those parts that actually are needed.

Answer 4

When you link to a static library the linker pulls in things that resolve names used in other parts of the code. In general, if the name isn't used it doesn't get linked in.