C++ Compilation and Linking

The compilation of a C++ program involves three steps:

1. Preprocessing: the preprocessor takes a C++ source code file and deals with the #include, #define and other preprocessor directives. The output of this step is “pure” C++ file without pre-processor directives.
2. Compilation: the compiler takes the pre-processor’s output and produces an object file from it.
3. Linking: the linker takes the object files procduced by the compiler and produces either a library or an executable file.

Preprocessing

The preprocessor handles the preprocessor directives, like #include and #define. It is agnostic of the syntax of C++, which is why it must be used with care.

It works on one C++ source file at a time by replacing #include directives with the content of the respective files (which is usually just declarations), doing replaccement of macros (#define), and selecting different portions of text depending of #if, #ifdef and #ifndef directives.

The preprocessor works on a stream of preprocessing tokens. Macro subsititution is defined as replacing tokens with other tokens (the operator ## enables merging two tokens when it makes sense).

After all this, the preprocessor produces a single output that is stream of tokens resulting from the transformations described above. It also adds some special markers that tell the compiler where each line came from so that it can use those to produce sensible error messages.

Some errors can be produced at this stage with clever use of the #if and #error directives.

Compilation

The compilation step is performed on each output of the preprocecssor. The compiler parses the pure C++ source code (now without any preprocessor directives) and convert it into assembly code. Then invokes underlying back-end (assembler in toolchain) that assembles that code into machine code producing actual binary file in some format (ELF, COFF, a.out, …). This object file ontains the compiled code (in binary form) of the symbols defined in the input. Symbols in object files are referred to by name.

Object files can refer to symbols that are not defined. This is the case when you use a declaration, and don’t provide a definition for it. The compiler doesn’t mind this, and will happily producce the object fle as long as the source code is well-formed.

Compiler usually let you stop compilation at this point. This is very useful because with it you an compile each source code file separately. The advantage this prodives is that you don’t need to recomiple everything if you only change a single file.

The produced object files can be put in special archives called static libraries, for easier reusing later on.

It’s at this sage that “regular” compiler errors, like syntax errors or failed overload resolution errors, are reported.

Linking

The linker is what produces the file compilation output from the object files the compiler produced. This output can be either a shared (or dynamic) library (and while the name is similar, they haven’t got much in common with static libraries mentioned earlier) or an executable.

It links all the object files by replacing the references to the undefined symbols with the correct addresses. Each of these symbols can be defined in other object files or in libraries. If they are defined in libraries other than the standard library, you need to tell the linked about them.

At this stage the most common errors are missing definitions or duplicate definitions. The former means that either the definitions don’t exist (i.e. they are not written), or that the object files or libraries where they reside were not given to the linker. The latter is obvious: the same sybmol was defined in two different object files or libraries.

Reference: https://stackoverflow.com/questions/6264249/how-does-the-compilation-linking-process-work