unresolved external

[r abbasi]

Hello all!
This is my header code, my1.h:

extern int t;
void print(int);
void print_t();

And this the my2.cpp code:

#include <my1.h>
#include <iostream>

void print(int i)
{
cout << i << endl;
}

void print_t()
{
cout << t <<endl;
}

And this is my use.cpp to uses those files:

#include <my1.h>
#include <conio.h>

int main()
{
t = 12;
print(99);
print_t();
_getch();
return 0;
}

for the use.cpp file I have added the my2.cpp file to that.

When I run the use.cpp, the following errors will appear:

Error 3 error LNK1120: 1 unresolved externals C:\Users\tom\documents\visual studio 2010\Projects\use\Debug\use.exe 1

Error 1 error LNK2001: unresolved external symbol "int t" (?t@@3HA) C:\Users\tom\documents\visual studio 2010\Projects\use\use\my2.obj

Error 2 error LNK2001: unresolved external symbol "int t" (?t@@3HA) C:\Users\tom\documents\visual studio 2010\Projects\use\use\use.obj

My machine in Windows 7, and I use MS Visual studio 2010 IDE.
Is there anyone to help me solve those errors please?

[Fransje]

change #include <my1.h> to #include "my1.h"

[r abbasi]

I did that but errors still exist as before!

[Fransje]

oh, now I understand: you tell the compiler with extern int t; that in another file int t exists, but it doesn't. You should add int t; to my2.cpp or use.cpp.

[r abbasi]

Sorry I can't understand! I don't have int t in another file, and in addition if I declare an extern int with another name (say, extern int foo) the problem still exists and only the name of 't' will replace with 'foo' in error messages.

I think whatever problem there is it's about extern int t
I declared the extern int t in my1.h and defined it in my2.cpp and used in use.cpp. It should works easily but doesn't!

[cire]

Sorry I can't understand! I don't have int t in another file,

What you have to understand is that you need int t; in another file, because currently you have no definition of the variable, only a declaration.

In "header.h"

// ...
extern int t ; // a declaration of t.
// ... 

In "header.cpp"

#include "header.h"
// ...
int t = 0 ;   // The definition of t should be at global scope.
// ... 

In "main.cpp"

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

int main()
{
    std::cout << t << '\n' ;
}

[tmihos]

I think you want to pass the value of t on the print_t() function. In this case t must contained on print_t() arguments as:

void print_t(int t){
  cout<<t<<'\n';
}

In this case not use external for t. This is common variable passing by value.
External used for precompiled libraries to .obj. #include works like included code has writen in the include point.

[r abbasi]

Thank you for all you nice guys, especially cire. The problem solved. thank you again.