C++ inline keyword

inline keyword 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.

This example shows inline functions in headers avoiding ODR violations.

// 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