dev-cpp-users

Re: [Dev-C++] Struckts and pointer .......

[<Met...@ao...>]

Did you allocated the memory needed by the data element pointed to by the 
struct pointer ?

Ie :

typedef struct _s
{
  int x,y;
  struct -s *next=NULL;
}

struct _s foo;
struct_s bar;

If you wish to make the foo.next elemnt to point to bar, you must :

foo.next=(struct _s*)malloc(sizeof(struct _s));
foo.next=bar;

Kind regards

--

Seb

Re: [Dev-C++] Struckts and pointer .......

[<Met...@ao...>]

mu...@gm... wrote :

> In what you do here, you're overwriting the pointer to the memory allocated,
> causing a memory leak.

Yes indeed !

In fact, I was thinking of one of my program I am currently developping. It 
makes intensive use of chained lists like this :

typedef struct _coldesc
{
  SQLCHAR ColumnName[255];
  SQLSMALLINT NameLength;
  SQLSMALLINT DataType;
  SQLUINTEGER ColumnSize;
  SQLSMALLINT DecimalDigits;
  SQLSMALLINT Nullable;
  TCHAR *buffer;
  SQLLEN IndPtr;
  struct _coldesc *next;
}; 

_coldesc *thisbinding=NULL;
_coldesc *firstbinding=NULL;
_coldesc *nextbinding=NULL;

firstbinding=(_coldesc*)malloc(sizeof(_coldesc));
thisbinding=firstbinding;

**Do calls to SQLDescribeCol here**

nextbinding=(_coldesc*)malloc(sizeof(_coldesc));
thisbinding->next=nextbinding;
thisbinding=thisbinding->next;

Sorry for the misunderstanding ! This small code example shows better what I 
was trying to tell.

--

Seb

Re: [Dev-C++] Struckts and pointer .......

[Abhijit S. <mu...@gm...>]

> Did you allocated the memory needed by the data element pointed to by the
struct pointer ?
>
> Ie :
>
> typedef struct _s
> {
>  int x,y;
>  struct -s *next=NULL;

Typo: struct_s *next = NULL, right?

> }
>
> struct_s foo;
> struct_s bar;

Remember this line...

>
> If you wish to make the foo.next elemnt to point to bar, you must :
>
> foo.next=(struct _s*)malloc(sizeof(struct _s));
> foo.next=bar;

In what you do here, you're overwriting the pointer to the memory allocated,
causing a memory leak.

It's like saying:

    b = 25;
    a = 35;
    a = b;

That's dangerous and unnecessary, since `bar' is already allocated on the
stack on the line marked. So you would just have to say

    foo.next = &bar;

Only if `bar' is a pointer would you do this:

    struct_s *bar;
    bar = new struct_s;
    foo.next = bar;

Regarding the original problem, of filling up a pointer to a struct, it
would be like this:

    struct_s *foo = new struct_s;
    foo->value = 25;

This will not cause a crash (provided `new' allocated).

Also note that `malloc' is considered dangerous (since it doesn't call the
constructor), and only `new' should be used. However, for allocating buffers
(of chars, etc.), I find it better to use malloc() and realloc().

One more thing, it's always better to initialize all pointers to NULL, so
that any error in your code will result in an immediate crash with a
NULL-pointer error, rather than a hidden bug that'll surface on the other
side of the program and make debugging a nightmare.

[Dev-C++] stl accumulate question

[<h.r...@gm...>]

Hi list,

i want to replace the following code:

typedef vector<double>::iterator it;
  
for(it i = TMW.begin(); i != TMW.end(); i++) { 
  sx   += *i;
  sxx += sqr(*i); 
}
    
with
  
    sx  = accumulate(TMW.begin(), TMW.end(), 0.0);
    sxx = accumulate(TMW.begin(), TMW.end(), 0.0, sqr);
[...]

def of sqr:

template<class T> 
  inline T sqr(T x) { return x * x; }

but sxx = .... does not compile.

The error message:
55 E:\dev\dev-c++\proj2\procabi.cpp
no matching function for call to `accumulate (double *, double *, double,
{unknown type})'

Why that ???

Thanks in advance

greetings
Horst    

-- 
GMX - Die Kommunikationsplattform im Internet.
http://www.gmx.net

RE: [Dev-C++] stl accumulate question

[Ioannis V. <no...@ho...>]

> -----Original Message-----
> From: dev...@li... 
> [mailto:dev...@li...] On Behalf 
> Of h.r...@gm...
> Sent: Thursday, September 05, 2002 1:20 AM
> To: dev...@li...
> Subject: [Dev-C++] stl accumulate question
> 
> 
> Hi list,
> 
> i want to replace the following code:
> 
> typedef vector<double>::iterator it;
>   
> for(it i = TMW.begin(); i != TMW.end(); i++) { 
>   sx   += *i;
>   sxx += sqr(*i); 
> }
>     
> with
>   
>     sx  = accumulate(TMW.begin(), TMW.end(), 0.0);
>     sxx = accumulate(TMW.begin(), TMW.end(), 0.0, sqr);
> [...]
> 
> def of sqr:
> 
> template<class T> 
>   inline T sqr(T x) { return x * x; }
> 
> but sxx = .... does not compile.
> 
> The error message:
> 55 E:\dev\dev-c++\proj2\procabi.cpp
> no matching function for call to `accumulate (double *, 
> double *, double,
> {unknown type})'
> 
> Why that ???


#include <numeric>



Ioannis

* Ioannis Vranos
* Programming pages: http://www.noicys.cjb.net
* Alternative URL: http://run.to/noicys