Inline specification on ordinary functions: Is it needed on the declaration, definition or both? [duplicate]

问题

This question already has answers here:

When should I write the keyword 'inline' for a function/method? (13 answers)

inline in definition and declaration (2 answers)

Closed 4 years ago.

 #include <iostream>
 using namespace std;

 void f();

 int main(){
      f();
 }

 void f(){
     cout << "Hello Stackoverflow!" << endl;
 }

If I wanted to succeed in making this function inline, would I need to specify inline in the declaration or definition, or both? I know that inline functions are normally defined in headers so normally there shouldn't be a separate declaration, but I was curious about the few unique cases where there are.

来源：https://stackoverflow.com/questions/32020122/inline-specification-on-ordinary-functions-is-it-needed-on-the-declaration-def