C++ modifying struct values from function by reference (potentially compile order issue)

Question

I've been doing a fair amount of reading on passing a struct member to a function. However if someone could clarify where I am going wrong I would appreciate it. I predict this is more a compile order / definition error than anything, although there may be syntax errors also.

As I understand it, I am able to modify the struct members data, and avoid any copying by passing as a reference (in this case NOT a pointer). This method will also allow me to keep the dotted notation for accessing individual members inside the function.

Main.cpp:

// include headers
struct foo{
  int a;
  int b;
}foo_data;

int main(){
  foo_data.a = 1; // assign some values to the member in standard format.
  foo_data.b = 2;

  function(foo_data); 
  // should this be &foo_data, am I passing by value here as it is
  // or is the declaration taking care of that? 
}

functions.h:

void function(foo &) // function declaration reference of type foo

functions.cpp:

void function(foo &foo_data) // function definition reference of type foo
{
  foo_data.a; // should == 1
  foo_data.b; // should == 2
}

The errors typically relate to undeclared identifiers, being foo_data. Do I need to tell tell the declaration in functions.h I am passing it a struct, or in fact an int (reference?)

Thanks!

Answer 1

Root Cause:
You get the error because compiler does not know/understand the type foo which you use in your cpp file. It can know the type only if it sees its definition.

Solution:
You need to put the declaration of the structure in a header file and include that header file in both the header where you declare the function and the cpp file where you define it.

functions.h

struct foo{
int a;
int b;
};
extern foo foo_data;
void function(foo &);

functions.cpp

#include "functions.h"
foo foo_data;


void function(foo &foo_data) function definition reference of type foo
{
    //access local foo_data
    foo_data.a // should = 1
    foo_data.b // should = 2

    //access global foo_data
    ::foo_data.a = 1;
    ::foo_data.b = 1;
}

Main.cpp

#include "functions.h"

int main()
{
    //Global foo_data is accessible here now
    foo_data.a = 
    foo_data.a =
    ....
}

EDIT:
Your comments suggest you want to share a foo_data instance across multiple source files, in that case you should use extern, I modified the above code sample to do so, note that inside function() the local foo_data instance will hide the global instance and to if you want to use the global object then you have to use it as ::foo_data.

Answer 2

The class definition for foo must be visible to both main and function. Do it like this:

// File foo.hpp

struct foo { int a; int b; };
void function(foo &);

// File main.cpp

#include "foo.hpp"

int main()
{
    foo foo_data;
    foo_data.a = 1;
    function(foo_data);
}

// File foo.cpp

#include "foo.hpp"

void function(foo & x)
{
     // use x.a, x.b etc.
}

If your function only needs to read the values, you can declare it with a foo const & argument.