inline

Learn the inline keyword and linker impact.

Inline

The inline keyword allows a function to be defined in multiple translation units; it relaxes the ODR (One Definition Rule). The compiler may replace a function call with the body, but this decision belongs to the compiler.

In modern C++ inline is not really a speed hint but an ODR relaxant. The compiler already makes the best decision at -O2 and above. __attribute__((always_inline)) (GCC/Clang) forces inline; but misuse bloats code and hurts cache friendliness.

ODR relaxation

Normally a function can have only one definition (ODR). But inline functions may be defined in multiple translation units; the linker merges them. This lets you put function definitions in a header file.

Example: in a utils.hpp header you write inline int square(int x) { return x * x; }. Two .cpp files include this header; both see the definition and no link error occurs. Without inline this would be an ODR violation.

Exercise Write the inline square function

Write inline int square(int x) with body return x * x;. In main write std::cout << square(6);.

Show example solution

#include <iostream>

inline int square(int x) {
    return x * x;
}

int main() {
    std::cout << square(6);
    return 0;
}

Test yourself

What does the inline keyword do in modern C++?
```
inline int square(int x) { return x * x; }
```
- Only speeds things up
- Deletes the code
- Removes classes
- Relaxes ODR, compiler decides on inlining
Show answer

Relaxes ODR, compiler decides on inlining — inline relaxes ODR; the speed decision belongs to the compiler.

Complete the inline function

inline int square(int x) { return x * ____; }

Show answer

inline int square(int x) { return x * x; }

Match each tool to its role

inline

ODR relaxation
__attribute__((always_inline))

Forced inline
-O0

Inlining off
-O2

Inlining on

Next lesson: Move vs copy

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