I have been trying to use extern in order to use variable that is previously defined.
I have not used extern before and now I need to use it in order to define variable just once and use them across multiple files
I have written minimized version of code for this question. I have four files
lib.h
#ifndef LIB_H
#define LIB_H
#include <iostream>
namespace lib {
extern bool initialized;
bool initialized = false;
static void isInit(char* parent) {
std::cout << "Library for [" << parent << "] initialized? " << (::lib::initialized ? "yes" : "no") << "\n";
}
} // namespace lib
#endif
vehicle.h
#ifndef _VEHICLE_H
#define _VEHICLE_H
#include <string>
class Vehicle {
public:
Vehicle(const std::string& manufacturer,
const std::string& model,
int year);
std::string manufacturer;
std::string model;
int year;
};
#endif
Following is implementation of vehicle.h file called vehicle.cpp
#include "vehicle.h"
#include "lib.h"
Vehicle::Vehicle(const std::string& manufacturer,
const std::string& model,
int year) :
manufacturer(manufacturer),
model(model),
year(year) {
::lib::isInit("Vehicle");
}
main.cpp
#include "vehicle.h"
#include "lib.h"
int main(int argc, char** argv) {
::lib::isInit("main");
::lib::initialized = true;
::lib::isInit("main");
Vehicle vehicle("Toyota", "Corolla", 2013);
return 0;
}
I am using g++
g++ -Wno-write-strings main.cpp vehicle.cpp -o bin/main.cpp.bin
I get following errors:
/tmp/cclVpsgT.o:(.bss+0x0): multiple definition of `lib::initialized'
/tmp/ccmJKImL.o:(.bss+0x0): first defined here
collect2: error: ld returned 1 exit status
I have checked the output of:
g++ -Wno-write-strings main.cpp vehicle.cpp -E
multiple definition occurs every time lib.h is included.
My questions are:
- Why is lib.h included multiple times when define guard is there
- How would I define 'extern' variable and initialize it in the same file (since it's used in the same file later)