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# C++ inline keyword

**[inline keyword](https://en.cppreference.com/w/cpp/language/inline)** marks a function to permit multiple definitions across translation units and allows the compiler to consider inlining (replacing the call with the function body). Today, `inline` is mainly for allowing definitions in headers; the actual inlining decision is made by the optimizer regardless of the keyword.

Use `inline` on function definitions in headers, or use `constexpr` and templates which are implicitly inline.

## Example

This example shows inline functions in headers avoiding ODR violations.

```cpp
// compile: g++ -o inline inline.cpp math.cpp
// run: ./inline
// description: inline allows function definitions in headers

#include <iostream>

// Allowed in header due to inline
inline int square(int x) { return x * x; }

// Modern alternative: constexpr (implicitly inline)
constexpr int cube(int x) { return x * x * x; }

int main() {
    std::cout << "square(5): " << square(5) << "\n";
    std::cout << "cube(3): " << cube(3) << "\n";
    
    return 0;
}

// math.cpp can define other inline functions from the same header
```