Header files w/ c++, specifically data structures

[iochinome]

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!

[Lindley]

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

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++.

[iochinome]

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

[Amleto]

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.

[Richard.J]

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

[iochinome]

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

#ifndef FUNCTIONS_H
#define FUNCTIONS_H

struct people {
     int age;
     bool gender;
};


#endif

functions.cpp:

#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:

#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!

[Lindley]

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

people person1 = {20, true};

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

people person1(20, true);

[jefranki]

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:

#ifndef FUNCTIONS_H
#define FUNCTIONS_H

struct people {
     int age;
     bool gender;
};

void printPerson(const struct people &thePerson);

#endif

functions.cpp:

#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

#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;
}