inline and the ODR

Inlining vs Linkage

inline is a hint for inlining, but more importantly it relaxes the One Definition Rule (ODR) so the same function definition can appear in multiple translation units (e.g., in a header).

// utils.hpp
#pragma once
inline int add(int a, int b) { return a + b; } // legal to define in a header

Notes

• Compilers may inline regardless of the keyword.

• Avoid inlining large, recursive, or virtual functions for performance reasons.