CODEWITHSUNDEEP
c++structlinkerglobal
zeno
zeno

Reputation: 81

Global struct c++

I'm aware that this question has been asked multiple times before, but I can't get this to work. For a special case I need to have a global struct that will be filled in one file, and read in another.

The following code gives me a linking error:

Undefined symbols for architecture x86_64:
  "_camera_data", referenced from:
      MyCameraData::return_sensor_shift() in extra-424af5.o
ld: symbol(s) not found for architecture x86_64
clang-5.0: error: linker command failed with exit code 1 (use -v to see invocation)

common.h:

#ifndef COMMON_H
#define COMMON_H
struct MyCameraData
{
    float sensor_shift;
    float return_sensor_shift();
};
extern MyCameraData camera_data;
#endif

main.cpp:

#include "common.h"
#include <iostream>

int main(){
    MyCameraData camera_data;

    camera_data.sensor_shift = 5.0;

    std::cout << camera_data.return_sensor_shift() << std::endl;
}

extra.cpp:

#include "common.h"

float MyCameraData::return_sensor_shift(){
    return camera_data.sensor_shift;
}

There must be something I'm overlooking. Any ideas?

Upvotes: 0

Views: 2335

Answers (2)

Francis Cugler
Francis Cugler

Reputation: 7905

This part is okay:

Common.h

#ifndef COMMON_H
#define COMMON_H
struct MyCameraData
{
    float sensor_shift;
    float return_sensor_shift();
};
extern MyCameraData camera_data;
#endif

Since you declared camera_data as extern in the header file which is only a declaration you are missing its definition.

To fix this add this file:

Common.cpp

#include "Common.h"
MyCameraData camera_data;

And that should fix your problem.

Upvotes: 2

R Sahu
R Sahu

Reputation: 206577

You could move

MyCameraData camera_data;

from main to global scope in main.cpp to resolve the linker error. However, I suggest creating a file common.cpp and defining the variable in that file. That is cleaner than defining it in main.cpp.

common.cpp:

#include "common.h"
MyCameraData camera_data;

main.cpp:

#include "common.h"
#include <iostream>

int main()
{
    camera_data.sensor_shift = 5.0;
    std::cout << camera_data.return_sensor_shift() << std::endl;
}

Upvotes: 2

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