Compilation phases

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.

Exercise Use a #define macro

Write #define ANSWER 42. In main write std::cout << ANSWER;.

Show example solution

#include <iostream>

#define ANSWER 42

int main() {
    std::cout << ANSWER;
    return 0;
}

Test yourself

In which phase are #include directives resolved?
```
#include <iostream>
int main() { ... }
```
- Preprocess
- Compile
- Link
- Run
Show answer

Preprocess — Preprocessing handles #include and #define directives.
Complete the #define name
```
#define ____ 42
```
Show answer
```
#define ANSWER 42
```
Match each phase to its role

preprocess

#include handling
compile

.cpp → .s (assembly)
assemble

.s → .o (object)
link

.o → executable

Next lesson: AST

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