C++ Compilation process

The process of converting human-readable C++ code into an executable machine program involves multiple stages. Each stage plays a critical role in ensuring that our code runs as intended.

We'll walk through the entire C++ compilation process, step-by-step, from source code to executable.

Steps involved in Compilation:

1. Source Code (.cpp, .h files)
2. Preprocessing
3. Compilation
4. Assembly
5. Linking
6. Execution

1. Source Code

Source code is the C++ code that the programmer writes. It usually consists of:

• .cpp files: contain function definitions and main()
• .h files: contain function declarations or constants (headers)

Example:

// main.cpp
#include <iostream>
#include "math_utils.h"

int main() {
    int result = add(10, 5);
    std::cout << "Result: " << result << std::endl;
    return 0;
}

2. Preprocessing

The preprocessor handles lines that start with # (like #include, #define, etc.) and prepares the code for the compiler. It:

• Replaces #include <....> with actual code from the included files.
• Expands macros
• Handles conditional compilation (#ifdef, etc.)

Command:

g++ -E main.cpp -o main.i

Output:

The output (main.i) contains pure C++ code with all header files inserted and macros expanded.

Example:

#include <iostream>  // becomes --> contents of <iostream> inserted
#include "math_utils.h"  // becomes --> int add(int a, int b);

3. Compilation

The compiler translates preprocessed C++ code into assembly language, which is human readable low-level code, It :

• Does Syntax checking
• Converts C++ to Assembly code (.s file)

Command

g++ -S main.i -o main.s

Output

main.s contains low-level assembly instructions.

Example:

C++

int result = add(10, 5);

Assembly

movl $10, -4(%rbp)
movl $5, -8(%rbp)
call add

4. Assembly

The assembler converts the assembly code into object code, which is machine-readable binary (.o file).

Command

g++ -c main.cpp -o main.o

Output

A .o (object file) that contains machine code, not yet executable on its own

5. Linking

The linker combines multiple object files (main.o, math_utils.o) and links in external libraries (like iostream), creating a final executable. It:

• Connects the main() function in main.o to add() in math_utils.o
• Resolves symbols from the standard library

Command

g++ main.o math_utils.o -o main

Output

An executable file (usually called main or whatever name you give)

6. Execution

It is the final executable produced by the linker.

Command

./main

Output

Result: 15

Tools Involved in the Compilation Pipeline

Common Errors at Each Stage