C / C++ / MFC

char* p and char *p ....Difference ?

11-Mar-07 17:20

Stephen Hewitt wrote:
All the source code for MFC comes with your compiler.

Actually it does not, the majority of it comes with the compiler. I learned this by digging very deeply into the code trying to find the source of a particular piece of information and ran up against a road block. That is the ultimate source of the information was outside of the code given us to look at.

INTP
"Program testing can be used to show the presence of bugs, but never to show their absence."Edsger Dijkstra

Software_Specialist11-Mar-07 14:00

11-Mar-07 14:00

Hi all,
i am having this lil doubt over pointers. In some books and links its said that its same i.e both means that it is pointer to character which holds the contents of the character type variable.
Please let me knw what is correct. I am so confused about this issue...
Thanks in advance...

Mike_V11-Mar-07 14:10

Mike_V

11-Mar-07 14:10

You are right. Both are the same.

It is just a stylistic difference. Proponents of char* p say it emphasizes that the type of p is a pointer to a character. Proponents of char *p say it minimizes confusion with the syntax of C/C++. For example:

char* p, n, x;       // p is a char*, n and x are chars
char *p, n, x;      // same thing, but clearer

char* p, n, *x;     // what's that second * doing?  -- confusing
char *p, n, *x;     // clear that p, x, are now pointers.

If you can't tell, I prefer the "char *p" syntax.

HTH,
Mike

Software_Specialist11-Mar-07 14:23

11-Mar-07 14:23

ok gr8...
Now its making some sence ....
What do you understand by the below piece of code ..

class linklist
{
private:

struct node
{
int data;
node *link;
}*p;

So here link is a variable of type structure and node *link gives out the contents of the address of type struct node ..that means its giving the value stored in link. But what is this *p after the close bracket.
Pls correct me if i said something wrong....and pls tell me about this *p...
Thanks a lot

Mike_V11-Mar-07 13:44

Mike_V

11-Mar-07 13:44

CS assignment? Wink | ;)

Link is defined as a "node *" -- remember, node* p and node *p are equivalent. So that would not make link a variable of type structure but of type pointer to structure instead.

The *p is a shortcut syntax in C... it declares p to be a pointer to a node.

This is why you must place a semicolon after a struct or class declaration - otherwise it treats the struct {} as a type and the next token the variable name. So you could do:

struct {int x, y;} pt;

And then later you could access pt.x and pt.y. The type of pt would be called an "anonymous struct" since it has no name. I could have written:

struct POINT {int x, y;} pt;

and then pt would be a POINT structure.

Mike

John R. Shaw11-Mar-07 15:22

11-Mar-07 15:22

That was I little confusing even for me. Let me clarify a little bit for ‘Software_Specialist’.

pt is a variable of type ‘struct POINT’.

INTP
"Program testing can be used to show the presence of bugs, but never to show their absence."Edsger Dijkstra

Mike_V11-Mar-07 15:26

Mike_V

11-Mar-07 15:26

ah... right.. Thanks.

Sorry about that.

John R. Shaw11-Mar-07 16:32

11-Mar-07 16:32

Cool! I just remember how simple concepts such as that where very confusing. We may take them for granted, but any one who ask such a question needs clarification.

INTP
"Program testing can be used to show the presence of bugs, but never to show their absence."Edsger Dijkstra

jfg211-Mar-07 15:08

jfg2

11-Mar-07 15:08

1. The prior replies were correct, the two forms are the same.
I like it to the left because then it says what it is, a pointer to some type, char* (pointer to a char, or a string of chars), or a pointer to an int: int*; also, when you do a typecast ( "(char*)" )it looks like this:
char* pointer =(char*) malloc(...); -- since the malloc() function returns a pointer to a chunk of memory. See the part about deferencing below, for yet another reason to shift * to the left.

2. However, as far as having several variables declared in one line, you can do it, but, in general, style guides typically suggest that in modern code, there should only be one (not more than one) declaration per line of code:
char c = '\0';
char* p, i, x; is confusing -- the * only applies to p, not i or x.

// as an extra here in this example, I'm initializing the variable 'c' to equal the 'null terminator,' ('\0'), that is used by the C language to terminate (i.e., end, all strings); also called a sentinel. The C run-time libraries look for this character to detect that the string has ended.

3. Next, you can declare a pointer (pChar) to point to the character (c):
char* pChar = &c;

Here, I'm initializing the pointer to have a value set equal to the address of the variable 'c'. The "&" operator returns the address of what follows it. The pointer variable 'pChar' has the special job of holding the address
of another variable's address, that is, where it 'lives' (resides) in memory. This can be used as one way to prevent crashes, due to uninitialed vars.

4. Now you can do this

char next = *pChar;

And next will have the value also held by 'c' the null ('\0'), which I could have set to any char, such as 'a', 'b', etc. When the asterisk is applied to the initialized pointer, in this way *p, then it is being "dereferenced" meaning that you are going to get the value of the variable the p hold the address of -- yes, it's called indirection, and can be confusing. But this give C the power to do many amazing things, when done right.

5. The struct definition in C let's you declare a variable (as simple struct object, or as a pointer to one of those struct objects, that you are defining the struct to represent, when you define the struct (the 'p' in this case, if I recall correctly) -- this is somewhat strange in that most other languages don't allow for this.

Your text book should show many examples of the linked list. That stuff gets very complex, very quickly, so get someone to show/explain it (fast!) and save yourself hours/days/weeks of pain and strain -- it is much harder than you think, at first. Roll eyes | :rolleyes:

Good Luck!! Tons of examples exist in the net: Checkout: http://www.cppworld.com/tutorials/?tutorial=structures

Software_Specialist12-Mar-07 3:27

12-Mar-07 3:27

so with in main there is no need to declare a object of struct if we want as p could access the data.
I mean there is no need to do -:
int main()
{
Point p1, p2;
because we can do something like this now i.e.
p.data, p.link;
Is that right.
Thanks a lot to all of you to make it so clear.

John R. Shaw11-Mar-07 16:43

11-Mar-07 16:43

Excellent! That will give the original poster much information to dwell on.

‘Mike_V’ was correct as I stated and your knowledge appears to be just as good. I am a teacher by nature, not by vocation, and therefore I like to clarify things when I can.

Why you where writing your post I was doing the same, so take a look and tell me what you think.

INTP
"Program testing can be used to show the presence of bugs, but never to show their absence."Edsger Dijkstra

John R. Shaw12-Mar-07 4:04

12-Mar-07 4:04

Interesting handle you gave yourself ‘Software_Specialist’, add in training to that.

You have gotten off the point of your original post; which had to do with pointers and not types.

In you little code snippet you declare the variables ‘p1’ and ‘p2’, the variable ‘p’ does not exist unless it was declared globally. The values stored in ‘p1’ can be accessed via ‘p1.data’ and ‘p1.link’, and the values stored in ‘p2’ can be accessed via ‘p2.data’ and ‘p2.link’.

You can create your own data type at any time and any where in your code, whether it is a ‘struct’ or a ‘class’.

int main()
{
struct MY_DATA_TYPE { int a, b, c; } my_data;
my_data.a = 1;
my_data.b = 2;
my_data.c = 3;
…..
}

INTP
"Program testing can be used to show the presence of bugs, but never to show their absence."Edsger Dijkstra

John R. Shaw11-Mar-07 16:22

11-Mar-07 16:22

The books you have read are correct, they are the same thing. I personally like writing it as char* p, but that is deceptive, as the symbol ‘*’ applies to (modifies) the variable ‘p’ and not the main type (char). The current trend is to declare each variable on an individual line to avoid confusion by those who do not understand that little detail. Once you understand what the ‘*’ symbol means in your code, then you can decide how best to express that meaning by its placement in your own code.

When declaring a variable with the modifier ‘*’, you are stating that the variable will be holding the address (location in memory) of the location where the value, or values, is actually stored. This is referred to as an indirect reference, meaning that it tells you where the value is stored and does not represent the value itself. When using a pointer variable you can use either ‘*p’ or ‘p->’ both of which is called a reference. A reference refers to the actual value(s) stored in memory and can be used to modify that value(s).

The simplest way I know how to explain it is this:
If I know the address of your house and can send you a letter at any time, then I know where you live (that is a pointer). If I go to your house as a guessed, then I can observer and affect the contents of your house, if I so choose (that is a reference).

That is enough, I only took one class on programming and that was because I was having trouble understanding the concept of pointers. Once I understood that the rest of it became non-relevant to me.

Good Luck!

INTP
"Program testing can be used to show the presence of bugs, but never to show their absence."Edsger Dijkstra

Software_Specialist12-Mar-07 3:38

cannot get edit box value (QWidget )

12-Mar-07 3:38

Thanks a lot to all of you.
Now its making much sence and i could now continue with programming of Linked List based problem.

Bilge Kaan11-Mar-07 12:34

Bilge Kaan

11-Mar-07 12:34

hello,
I want to get edit box value from an application.I am using GetWindowText Api,but returning caption not the value written in edit box.Using spy++ , window class name is QWidget , window caption is something which is not visible.
Any idea?

Re: cannot get edit box value (QWidget )

John R. Shaw11-Mar-07 17:28

Build error LNK2001 on existing Project

11-Mar-07 17:28

Only one; look up QWidget which is a cross platform library for creating applications.

INTP
"Program testing can be used to show the presence of bugs, but never to show their absence."Edsger Dijkstra

JAG1811-Mar-07 11:09

JAG18

11-Mar-07 11:09

I am trying to modify an existing project in Visual C++ 6.0. (I do not do a lot of C++ programming, but am helping a friend.)

When I try and build a project, I receive some errors. There are 11 errors, but I have only copied two of them to shorten the post.

SUMMARYT.OBJ : error LNK2001: unresolved external symbol "protected: virtual void __thiscall CSummaryTableView::OnDraw(class CDC *)" (?OnDraw@CSummaryTableView@@MAEXPAVCDC@@@Z)
SUMMARYT.OBJ : error LNK2001: unresolved external symbol "public: __thiscall CTableViewEdit::CTableViewEdit(void)" (??0CTableViewEdit@@QAE@XZ)

This is an existing program that is already compiled, I just added a new field to the report. I have confirmed that the Constructors have been built. What else could be causing these errors?

Visual C++ is new to me, so I would appreciate some help. Thanks.

JAG

Re: Build error LNK2001 on existing Project

John R. Shaw11-Mar-07 17:38

11-Mar-07 17:38

I recommend ‘Rebuild All’, these problems usually occurs because a partial build relies on previous information that is no longer valid. If that does not work, then there is a missing code file (it could be corrupted) or an intermediate file is giving you grief. Check a few FAQs to be sure, but you may need to delete to ‘*.ncb’ file in order to force the compiler to recreate the entire code from scratch.

INTP
"Program testing can be used to show the presence of bugs, but never to show their absence."Edsger Dijkstra

export to text file

Kiethnt11-Mar-07 8:41

Kiethnt

11-Mar-07 8:41

Dear all,
Now I have six data in includes:
3 data Xw, Yw, Zw in FlexGid at i row and j column
and 3 data m_iA, m_iB, m_iC in edit box form Interger individually. So please help me export these data to text file .txt in which these data is arrange correctly following form:
P1={X Xw,Y Yw,Z Zw,A m_iA,B m_iB,C m_iC}
P2={X Xw,Y Yw,Z Zw,A m_iA,B m_iB,C m_iC}....Pn
I just start with VC++, so please show me more detail. Thank you very much.

Re: export to text file

John R. Shaw11-Mar-07 16:20