How do C++ header files work?

Question

When I include some function from a header file in a C++ program, does the entire header file code get copied to the final executable or only the machine code for the specific function is generated. For example, if I call std::sort from the <algorithm> header in C++, is the machine code generated only for the sort() function or for the entire <algorithm> header file.

I think that a similar question exists somewhere on Stack Overflow, but I have tried my best to find it (I glanced over it once, but lost the link). If you can point me to that, it would be wonderful.

Answer 1

You're mixing two distinct issues here:

1. Header files, handled by the preprocessor
2. Selective linking of code by the C++ linker

Header files

These are simply copied verbatim by the preprocessor into the place that includes them. All the code of algorithm is copied into the .cpp file when you #include <algorithm>.

Selective linking

Most modern linkers won't link in functions that aren't getting called in your application. I.e. write a function foo and never call it - its code won't get into the executable. So if you #include <algorithm> and only use sort here's what happens:

• The preprocessor shoves the whole algorithm file into your source file
• You call only sort
• The linked analyzes this and only adds the source of sort (and functions it calls, if any) to the executable. The other algorithms' code isn't getting added

That said, C++ templates complicate the matter a bit further. It's a complex issue to explain here, but in a nutshell - templates get expanded by the compiler for all the types that you're actually using. So if have a vector of int and a vector of string, the compiler will generate two copies of the whole code for the vector class in your code. Since you are using it (otherwise the compiler wouldn't generate it), the linker also places it into the executable.