Header files w/ c++, specifically data structures

hey guys, so i am doing a project right now that requires that a specific data structure (there are a few actually, but its all the same case) be available/defined in multiple source files. i used to do my program via just including the .cpp source files in the main file, but now i am doing an overhaul- i am adding in header files and just including those. so what i have is:
2 .cpp files, main.cpp and functions.cpp. i made a header file for each, include both in main.cpp like so:
#include "main.h"
#include "functions.h"

in functions.h, i want to define the data structure. i did it like this:
struct people {
int age;
bool gender;
}

extern struct people person_1; /*an instance i want available across all source files*/
extern struct people person[5];

then, in main.cpp, i went like this:
struct people person_1;
struct people person[5];

it gave me a whole slew of errors, including:
error: aggregate ‘people person1’ has incomplete type and cannot be defined
error: elements of array ‘people person [5]’ have incomplete type
error: storage size of ‘person’ isn't known

whats up with it? how to fix? thanks!

Looks like you forgot the semicolon at the end of the struct definition:

Code:

---

struct people {
    int age;
    bool gender;
};

---

A couple of things, too. First, "data structure" has a slightly different connotation than you're using it to mean here. A data structure is a generic concept like a linked list, binary tree, trie, hash table, etc. Something like "people" you should simply refer to as a struct for clarity.

It's also unnecessary to use the struct keyword when declaring the global variables. That was necessary in C (unless you typedef'd it away), but not in C++.

hmm... even with the semi-colon, i get the same error. any ideas? thanks!

arent you meant to switch around where you have declared extern?

do you use people inside main? becuase you include main first so maybe that is causing a problem too.

do you really include function.h in main.cpp?
Your code fractions do not clearly show this.

here, ill give the full files:
functions.h:

Code:

---

#ifndef FUNCTIONS_H
#define FUNCTIONS_H

struct people {
    int age;
    bool gender;
};


#endif

---

functions.cpp:

Code:

---

#include <stdio.h>
#include <stdlib.h>
#include "functions.h"

extern struct people person1;
extern struct people person_array[5];

person1.age = 20;
person1.gender = 10;

int print() {
        printf("hello world!\n");
        return 0;
}

---

main.cpp:

Code:

---

#include <stdio.h>
#include <stdlib.h>
#include <iostream>
#include "functions.h"

int main() {
        std::cout << "person1.age: " << person1.age << "\n";
        std::cout << "person1.gender: " << person1.gender << "\n";
        print();
        return 0;
}

---

obviously thats not my actual project, but its all the relevant stuff. now when i go to compile this, i get the error:

error: expected constructor, destructor, or type conversion before ‘.’ token

on both lines in functions.cpp where i attempt to assign person1.age and person1.gender. whats up? thanks guys!

The "extern" keyword should only be used in header files.

You can't do sequential statements (such as assigning values to age and gender) outside of a function. You can only do that in an initialization, such as

Code:

---

people person1 = {20, true};

---

Or, if you gave the people struct a constructor, you could do

Code:

---

people person1(20, true);

---

As of your last posting, there are several things wrong.

First of all, you are mis-using the extern keyword. extern does not define a variable, it only lets you re-use a global declared in some other cpp file within your project. Therefore, person1 is undeclared in main.cpp.

Second, there is really no good reason for person1 or person_array to be global in this project (generally avoid globals unless there is a really good reason to use them). They should be declared inside main(), then passed to other functions as needed, by reference if neccessary. Example:

functions.h:

Code:

---

#ifndef FUNCTIONS_H
#define FUNCTIONS_H

struct people {
    int age;
    bool gender;
};

void printPerson(const struct people &thePerson);

#endif

---

functions.cpp:

Code:

---

#include <stdio.h>
#include "functions.h"

extern const int someGlobal; //for extern illustration only

using namespace std;

void printPerson(const struct people &thePerson)
{
        cout<<"age: "<<thePerson.age<<", gender: "<<thePerson.gender;

}

---

main.cpp

Code:

---

#include <stdio.h>
#include "functions.h"

const int someGlobal=2; //for extern illustration only
 
using namespace std;

int main()
{
        struct people person1;

        person1.age=20;
        person1.gender=true;

        cout<<"Person1 - ";
        printPerson(person1);
        cout<<endl;

        return 0;
}

---