#pragma once
void my_func()
{
 ...
}

#include "header.h"
...
my_func();
...

#include "header.h"
...
my_func();
...

Error	LNK2005	"my_func()" (?my_func@@YAIPAXII@Z) already defined in first.obj

#pragma once
#ifdef INCLUDE_IN_OTHER_MODULE
void my_func();
#else
void my_func()
{
 ...
}
#endif

template< typename T >
void my_func()
{
 ...

Error	C2672	'my_func': no matching overloaded function found	
Error	C2783	'void my_func(void)': could not deduce template argument for 'T'

template< typename >
void my_func()

template< >
void my_func()

template <typename T>
void my_func(T x)
{
 ...

my_func(0);

void my_func()
{
  static int k = 0;
 ...

class X {
 void my_method() {
  ...
 }
 ...
}

inline void my_func()
{
  static int k = 0;
 ...

namespace {
	class Test {
		static int x;
	public:
		Test() {
			x++;
			printf("%d\n", x);
		}
	};

	int Test::x = 0;
}

extern int Test::x = 0;

int Test::x = 0;

#include <header.h>

class Test {
	int& x = get_x();

	int& get_x() {
		static int x = 0;
		return x;
	}

public:
	Test() {
		x++;
		printf("%d\n", x);
	}
};

class Test {
	int& x = get_x();

	static int& get_x() {
		static int x = 0;
		return x;
	}

public:
	Test() {
		x++;
	}

	static void print() {
		int& x = get_x();
		printf("%d\n", x);
	}
};

#pragma once