Source files, headers, and declarations | C++

C++ programs often span many files. The compiler does not compile the whole project as one giant text file. It compiles source files separately, then the linker connects the compiled pieces.

Source files

A .cpp file usually contains function and class definitions:

// math.cpp
int add(int a, int b) {
    return a + b;
}

When compiled, this file becomes an object file. Later, the linker can combine it with other object files.

Headers

A header usually contains declarations that other files need:

// math.h
#pragma once

int add(int a, int b);

Another source file can include that header:

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

int main() {
    std::cout << add(2, 3) << "\n";
}

The compiler can type-check the call because it has seen the declaration.

Declarations vs definitions

A declaration says what exists:

int add(int a, int b);

A definition creates it:

int add(int a, int b) {
    return a + b;
}

Most functions should be defined once. If the linker finds no definition, you get an unresolved symbol or undefined reference error. If it finds multiple conflicting definitions, you get a multiple definition error.

Include is textual

#include is handled before normal compilation. It effectively copies header text into the including file.

This is why headers should avoid unnecessary includes. If a header includes many other headers, every file that includes it may compile more slowly.

Introductory compile command

For a tiny multi-file program:

clang++ -std=c++20 -Wall -Wextra -pedantic main.cpp math.cpp -o app

This compiles both .cpp files and links them into one executable.

As projects grow, you will use CMake or another build system instead of typing every file manually.

What to carry forward

.cpp files are compiled separately
headers share declarations across files
declarations say what exists; definitions provide storage or code
linker errors are different from compiler errors
build systems become necessary when file counts grow

Next, you will study pointers and references, the first direct tools for talking about memory.