How do I point to structure members?

RaisinToe · 03-07-2009, 07:13 PM

I can not tell what is wrong with my code. I am just trying to learn how to point to structure members.

Code:

#include <iostream>
using namespace std;

struct structure
{
	char c;
	int  i;
	int a[40];
} str;

structure *p;
p = &str;

int main()
{
	str.a = 89;
	str.c = 89;
	str.i = 10;

	cout << p-> c;

	system("PAUSE");

	return 0;
}

grumpy · 03-07-2009, 07:39 PM

It is not possible to do assignments of pointers outside a function. Specifically,

Code:

    structure *p;
    p = &str;

is invalid outside a function. If you must have it outside any function, make it an initialisation.

Code:

    structure *p = &str;

There is a second problem, as "str.a = 89;" inside main() is invalid. This is because str.a is an array, so it cannot be an lvalue (i.e. it is not allowed on the left hand side of an assignment). This is one of those cases where - contrary to what a lot of beginners and quite a few textbooks tell you - an array and a pointer are not equivalent.

RaisinToe · 03-07-2009, 11:34 PM

Quote:

Originally Posted by grumpy

It is not possible to do assignments of pointers outside a function. Specifically,

Code:

    structure *p;
    p = &str;

is invalid outside a function. If you must have it outside any function, make it an initialisation.

Code:

    structure *p = &str;

Thank you, I have never heard anything about that before.

03-07-2009, 07:13 PM	#1
RaisinToe Registered User Join Date: Jan 2009 Posts: 31	How do I point to structure members? I can not tell what is wrong with my code. I am just trying to learn how to point to structure members. Code: #include <iostream> using namespace std; struct structure { char c; int i; int a[40]; } str; structure *p; p = &str; int main() { str.a = 89; str.c = 89; str.i = 10; cout << p-> c; system("PAUSE"); return 0; }
RaisinToe is offline

03-07-2009, 07:39 PM	#2
grumpy Registered User Join Date: Jun 2005 Posts: 1,343	It is not possible to do assignments of pointers outside a function. Specifically, Code: structure p; p = &str; is invalid outside a function. If you must have it outside any function, make it an initialisation. Code: structure p = &str; There is a second problem, as "str.a = 89;" inside main() is invalid. This is because str.a is an array, so it cannot be an lvalue (i.e. it is not allowed on the left hand side of an assignment). This is one of those cases where - contrary to what a lot of beginners and quite a few textbooks tell you - an array and a pointer are not equivalent.
grumpy is offline

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