Compilation phases
- C++ course
- Expert
- Lesson 37/47
- 5 min
Learn the C++ compilation phases.
Compilation phases
C++ compilation happens in four phases: preprocess, compile, assemble and link. Each phase takes the previous phase's output.
Preprocessing handles #include, #define and conditional compilation directives. Compilation turns the preprocessed source into machine-near assembly. Assembly turns the assembly into an object file (.o). Linking combines object files into an executable. This separation lets you understand which tool reports which error.
Phase details
Each phase's output is the next phase's input. Preprocessing turns a .cpp file into a .ii file; this file has all headers expanded. Compilation turns .ii into a .s (assembly) file. Assembly turns .s into a .o (object) file.
Linking takes multiple .o files, resolves symbols and produces an executable. g++ -E main.cpp does only preprocessing; g++ -S main.cpp goes up to compilation; g++ -c main.cpp goes up to assembly. These flags run each phase separately.
Practice Use a #define macro
Write #define ANSWER 42. In main write std::cout << ANSWER;.
Starter code
#include <iostream>
// Define the ANSWER macro here
int main() {
// Print ANSWER
return 0;
}Sample solution
#include <iostream>
#define ANSWER 42
int main() {
std::cout << ANSWER;
return 0;
}Expected output
42
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 In which phase are #include directives resolved?
#include <iostream>
int main() { ... }- Preprocess Correct answer
- Compile
- Link
- Run
Explanation: Preprocessing handles #include and #define directives.
2 Complete the #define name
#define ____ 42#define ANSWER 42Explanation: The ANSWER macro gets the value 42.
3 Match each phase to its role
| Term | Meaning |
|---|---|
preprocess |
#include handling |
compile |
.cpp → .s (assembly) |
assemble |
.s → .o (object) |
link |
.o → executable |