Can you define a static member function in a class?

Is there any regulation about where to define a static member function? My question is, can you define the static member function in a separated .cpp file? Or you have to define it in its .h file?

suppose you have a class in a test.h file

Code:

---

class A{
public:
  static void foo();
};

---

Then, in the main.cpp file, you include the test.h and give foo() a definition.

Code:

---

void A::foo(){
....//do something
}

---

It works in my PC, but my friend says that's incorrect.

Yes, you can put the implementation in a .cpp file.

In fact, you should. If you don't, and instead put the implementation in the .h file, you can't include the header file more than once, lest you get a linker error about defining the same function twice.

Code:

---

$ cat header.h
#ifndef HEADER_H
#define HEADER_H

#include <iostream>

class C {
public:
    static void function();
};

void C::function() {
    std::cout << "Hello, World!\n";
}

#endif
$ cat header.cpp
#include "header.h"

int main() {
    C::function();
    return 0;
}
$ cat header2.cpp
#include "header.h"

void another_function() {
    C::function();
}
$ g++ header.cpp header2.cpp -o header
/tmp/ccTTAxaC.o: In function `C::function()':
header2.cpp:(.text+0x11c): multiple definition of `C::function()'
/tmp/cca6d8S9.o:header.cpp:(.text+0x11c): first defined here
collect2: ld returned 1 exit status
$

---

Incidentally, you can put the method body in a header file if it's inline:

Code:

---

dwk@cypress:~/c/cb$ cat header.h
#ifndef HEADER_H
#define HEADER_H

#include <iostream>

class C {
public:
    static void function() { std::cout << "Hello, World!\n"; }
};

#endif
$ g++ header.cpp header2.cpp -o header
$ ./header
Hello, World!
$

---