Getting the basics of linking right

We discussed the life cycle of a C++ program in Chapter 7, Compiling C++ Sources with CMake. It consists of five main stages – writing, compiling, linking, loading, and execution. After correctly compiling all the sources, we need to put them together into an executable. We said that object files produced in a compilation can't be executed by a processor directly. But why?

To answer this, let's understand that object files are a variant of the widely-used Executable and Linkable Format (ELF), common in Unix-like systems and many others. Systems like Windows or macOS have their own, different formats, but we’ll focus on ELF to explain the principle. Figure 8.1 shows how a compiler structures these files:

Figure 8.1 : The structure of an object file

The compiler will prepare an object file for every unit of translation (for every .cpp file). These files will be used to build an in-memory image of our program. Object files consist...