[C++] passing custom type (structure) to a function

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Hi. I was experimenting with this and I found strange behaviour which I cant understand.
Consider this code:

C++

#include "stdafx.h"

void passStructAsParam(Student s);

struct Student
{
	int rollno;
	char gender;
	int age;
};



int main()
{
	Student s; 
	s.rollno = 40;
	s.gender = 'm';
	s.age = 20;

	passStructAsParam(s);

	return 0;
}

void passStructAsParam(Student s)
{
	printf("%d \n", s.rollno);
	printf("%c \n", s.gender);
	printf("%d \n", s.age);
}

This code wont work because the structure is defined after the function declaration but if I do this:

C++

#include "stdafx.h"

void passStructAsParam(struct Student s);

struct Student
{
	int rollno;
	char gender;
	int age;
};



int main()
{
	Student s; 
	s.rollno = 40;
	s.gender = 'm';
	s.age = 20;

	passStructAsParam(s);

	return 0;
}

void passStructAsParam(struct Student s)
{
	printf("%d \n", s.rollno);
	printf("%c \n", s.gender);
	printf("%d \n", s.age);
}

This works perfectly fine. So the question here is: Why when I explecitely write that the Student is a structure in the function's parameter list it works but if i dont it doesnt ?

What I have tried:

Asking question here in CodeProject.com

Posted 17-Jul-17 0:26am

The_Unknown_Member

Updated 17-Jul-17 2:19am

v2

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Comments

Richard MacCutchan 17-Jul-17 6:42am

What is the error that you see? Also, you should not pass complete structures as parameters, it is extremely wasteful. Pass by address or reference.

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Jochen Arndt · Accepted Answer · 2017-07-17T01:11:00

When using the struct keyword in your function declaration you have a so called "forward declaration":

// Forward declaration
// Definition must be present later in this scope 
struct Student;

// Can now use the forward declaration within other declarations
void passStructAsParam(Student s);
 
// Final definition of structure
struct Student
{
	int rollno;
	char gender;
	int age;
};

I have moved the forward declaration out of the function declaration to explain what is happening.