extern with arrays of structure

[sss555]

Hi,

I tried to share some data between 2 .cc files hence declared the arrays of structure as extern in one of the .cc files.

However I get abnormal entries in the original array entries otherwise I wouldn't have got.

file1.h
---------
struct Mylistdetails {
int id;
int value;
}


file1.cc
----------
struct Mylistdetails myList_[MAX_NO];


file2.cc
-----------
#include <file1.h>

extern struct Mylistdetails myList_[MAX_NO];


The purpose here is to call these array data into the file2.Here,myLi st_[i].id and myList_[i].value are just unexpected once declared as 'extern'.If not used this extern method the expected values are observed once only file2 is being used.

Could you kindly specify the behaviour of this.Many thanks for your advise on how to tackle such a problem.

[horace1]

did your code compile?
there is a ; missing in

struct Mylistdetails {
int id;
int value;
};  // missing ;

and when including a file in the current directory it should be in ""
e.g. file2.cc

#include "file1.h"
 
extern struct Mylistdetails myList_[MAX_NO];
 

you could put the extren declaration in the header file

[sss555]

did your code compile?
there is a ; missing in

struct Mylistdetails {
int id;
int value;
};  // missing ;

It compiled without any problem.


and when including a file in the current directory it should be in ""
e.g. file2.cc

#include "file1.h"
 
extern struct Mylistdetails myList_[MAX_NO];
 

you could put the extren declaration in the header file

well it is not in the current directory, so i specified the path
#include <direct/file1.h>

Yes I tried putting extern declaration in the heade file too(file1.h).St ill doesn't work.

Thanks for your attention.

[horace1]

well it is not in the current directory, so i specified the path
#include <direct/file1.h>

Yes I tried putting extern declaration in the heade file too(file1.h).St ill doesn't work.

Thanks for your attention.

I guess you are on a unix system, try

#include "direct/file1.h"

assuming you have a directory direct in your current directory which contains the file1.h

[sss555]

I guess you are on a unix system, try

#include "direct/file1.h"

assuming you have a directory direct in your current directory which contains the file1.h

I'm working on Linux, and it works normally in #include<direct/file1.h> style.

Could you explain me how could I use, extern with arrays of structure?

Thank you

[horace1]

#include <filename>                              // include file from 'standard' place 
 

the file is assumed to be in a 'standard' directory in the file system which the C/C++ compiler automatically searches,.

#include "filename"       // include file from current directory else 'standard' place 
 

The second form of #include (filename enclosed in "") searches the current directory for filename and then, if it is not found, the 'standard' directory.

which you use it depends where your direct/file1.h file is - in the 'standard' place or relative to the current directory?

try the following

// file1.h
 
struct Mylistdetails {
int id;
int value;
};
 
#define MAX_NO 10
 
extern struct Mylistdetails myList_[MAX_NO];
 
void setup();   // function prototype - function in file2.cpp
 

// file2.cpp
 
#include "file1.h"
 
void setup()
{
for(int i=0;i<MAX_NO; i++)
    myList_[i].id=i;
}
 

// file1.cpp
#include <cstdlib>
#include <iostream>
#include "file1.h"
 
using namespace std;
 
struct Mylistdetails myList_[MAX_NO];
 
int main(int argc, char *argv[])
{
    setup();
    for(int i=0;i<MAX_NO; i++)
        cout << myList_[i].id << endl;
    return EXIT_SUCCESS;
}
 

when i build this
g++ file1.cpp file2.cpp

and run it I get
./a
0
1
2
3
4
5
6
7
8
9

[sss555]

#include <filename>                              // include file from 'standard' place 
 

the file is assumed to be in a 'standard' directory in the file system which the C/C++ compiler automatically searches,.

#include "filename"       // include file from current directory else 'standard' place 
 

The second form of #include (filename enclosed in "") searches the current directory for filename and then, if it is not found, the 'standard' directory.

which you use it depends where your direct/file1.h file is - in the 'standard' place or relative to the current directory?

try the following

// file1.h
 
struct Mylistdetails {
int id;
int value;
};
 
#define MAX_NO 10
 
extern struct Mylistdetails myList_[MAX_NO];
 
void setup();   // function prototype - function in file2.cpp
 

// file2.cpp
 
#include "file1.h"
 
void setup()
{
for(int i=0;i<MAX_NO; i++)
    myList_[i].id=i;
}
 

// file1.cpp
#include <cstdlib>
#include <iostream>
#include "file1.h"
 
using namespace std;
 
struct Mylistdetails myList_[MAX_NO];
 
int main(int argc, char *argv[])
{
    setup();
    for(int i=0;i<MAX_NO; i++)
        cout << myList_[i].id << endl;
    return EXIT_SUCCESS;
}
 

when i build this
g++ file1.cpp file2.cpp

and run it I get
./a
0
1
2
3
4
5
6
7
8
9

In fact I want to retrieve file1 data into file2.If I understand your code correctly it does the other way round.

I tried this way and when I compile I get the following error

file1.h:270: error: storage class specified for 'myList_'



Thank you for your suggestions.

[horace1]

try this which passes data from file1.cpp to file2.cpp

// file1.h
 
struct Mylistdetails {
int id;
int value;
};
 
#define MAX_NO 10
 
extern struct Mylistdetails myList_[MAX_NO];
 
void setup();   // function prototype - function in file1.cpp
void print();   // function prototype - function in file2.cpp
 
 

// file2.cpp
 
#include <iostream>
#include "file1.h"
 
using namespace std;
 
 
void print()
{
    for(int i=0;i<MAX_NO; i++)
      cout << myList_[i].id << endl;
}
 
 

// file1.cpp
 
#include <cstdlib>
#include <iostream>
#include "file1.h"
 
using namespace std;
 
struct Mylistdetails myList_[MAX_NO];
 
void setup()
{
for(int i=0;i<MAX_NO; i++)
    myList_[i].id=i;
}
 
 
int main(int argc, char *argv[])
{
    setup();
    print();
    return EXIT_SUCCESS;
}
 

[sss555]

try this which passes data from file1.cpp to file2.cpp

// file1.h
 
struct Mylistdetails {
int id;
int value;
};
 
#define MAX_NO 10
 
extern struct Mylistdetails myList_[MAX_NO];
 
void setup();   // function prototype - function in file1.cpp
void print();   // function prototype - function in file2.cpp
 
 

// file2.cpp
 
#include <iostream>
#include "file1.h"
 
using namespace std;
 
 
void print()
{
    for(int i=0;i<MAX_NO; i++)
      cout << myList_[i].id << endl;
}
 
 

// file1.cpp
 
#include <cstdlib>
#include <iostream>
#include "file1.h"
 
using namespace std;
 
struct Mylistdetails myList_[MAX_NO];
 
void setup()
{
for(int i=0;i<MAX_NO; i++)
    myList_[i].id=i;
}
 
 
int main(int argc, char *argv[])
{
    setup();
    print();
    return EXIT_SUCCESS;
}
 

I still get this error while trying to compile.

file1.h:270: error: storage class specified for ‘myList_’



Thank you

[horace1]

I still get this error while trying to compile.

file1.h:270: error: storage class specified for ‘myList_’



Thank you

I have run the code in my last post on Linux and Windows (using Cygwin). I compile with
g++ file1.cpp file2.cpp

then execute the file a.out or a.exe OK