Inline

Inline is like static or extern. It’s a linker directive.

static means that the name cannot be used in other translation units. extern means that the name will be used by other translation units. inline means the function will be defined multiple times, and to use one definition.

In gcc and clang, generally the linker uses the first definition as the definition to use for all functions:

inline111.cpp

#include <iostream>

void bar();
inline int fun() { return 111; }

int main() {
  std::cout << "inline111: fun() = " << fun() << ", &fun = " << (void*) &fun;
  bar();
}

inline222.cpp

#include <iostream>

inline int fun() { return 222; }

void bar() {
  std::cout << "inline222: fun() = " << fun() << ", &fun = " << (void*) &fun;
}

fun() is declared twice here, but if you compile like so:

g++ -std=c++11 inline111.cpp inline222.cpp

The output is:

inline111: fun() = 111, &fun = 0x4029a0
inline222: fun() = 111, &fun = 0x4029a0

But if I swap them:

g++ -std=c++11 inline222.cpp inline111.cpp

The output is:

inline111: fun() = 222, &fun = 0x402980
inline222: fun() = 222, &fun = 0x402980

If the function is inlined away, however, the inline loses its meaning:

g++ -std=c++11 -O2 inline111.cpp inline222.cpp

inline111: fun() = 111, &fun = 0x402900
inline222: fun() = 222, &fun = 0x402900

You can use a compiler macro __attribute__((always_inline)); to force inlining.