G++ and LTO: how to separate declaration and definition?

问题

Since LTO can resolve inline symbols from other object files, I tried to separate declaration and definition for inline functions. Here is the 3 files, dep.hpp, dep.cpp and main.cpp, where main.cpp and dep.cpp are linked together and dep.hpp is #included in both:

// dep.hpp, declaration
#ifndef _DEP_HPP_
#define _DEP_HPP_
inline void sayHello();
#endif

// dep.cpp, definition
#include "dep.hpp"
#include <iostream>

inline void sayHello() {
    std::cout << "Hello World!" << std::endl;
}

// main.cpp, caller
#include "dep.hpp"

int main(int argc, char** argv) {
    sayHello();
}

And here's the error that G++ prints:

In file included from main.cpp:1:
dep.hpp: At global scope:
dep.hpp:4:13: warning: inline function 'void sayHello()' used but never defined
 inline void sayHello();
             ^~~~~~~~
C:\Users\vtonc\AppData\Local\Temp\ccojskQy.ltrans0.ltrans.o:<artificial>:(.text+0x15): undefined reference to `sayHello()'
collect2.exe: error: ld returned 1 exit status

The command line:

@echo off
g++ main.cpp dep.cpp -o example.exe -flto -Wall -Wextra
pause

Why is the linker complaining about undefined sayHello() when -flto is enabled and sayHello() itself is inline? Shouldn't LTO handle such cases?

I'm using mingw-w64 8.1.0 x86_64-posix-seh.

回答1:

The whole point of inline is that you must provide the function's definition in the same translation unit(s) in which you call the function.

Turning on a compiler optimisation doesn't change that; it's a rule of the language.

Move the definition to the header.

来源：https://stackoverflow.com/questions/57220421/g-and-lto-how-to-separate-declaration-and-definition