C / C++ / MFC

12-Feb-08 0:35

Hi to all,

I have some questions,

1. In which case there is need to overload 'new' & 'delete', gloabally or within a class?
2. How to check memory leak in an application developed in C/C++?
3. What is difference between a MACRO variable & const variable in C/C++? As we can pass or return const variables to/from functions, Can we do same with MACRO variable?
4. Is the 'Data Abstraction' is feature of C++? Then what do u mean by 'Data Abstraction'? Creating new data types? But we can create new data types in C using struct/union, then why 'Data Abstraction' is not feature of C?
5. Can we do the operation performed by calloc() using malloc()? Is it valid?
int *l = (int*) calloc(12, sizeof(int));
int *m = (int*) malloc(sizeof(int)*12);
6. Scope of static & global variables in an application?

Thanks in advance,

Regards,
Aniket A. Salunkhe

modified on Tuesday, February 12, 2008 7:20 AM

jhwurmbach12-Feb-08 1:30

jhwurmbach

12-Feb-08 1:30

Aniket Salunkhe wrote:
1. In which case there is need to overload 'new' & 'delete', gloabally or within a class?

Rarely.
Sometimes you want to have your allocation from a special heap memory.

Aniket Salunkhe wrote:
2. How to check memory leak in an application developed in C/C++?

In debug mode, the runtime does. It uses overloaded operator new for that!
Also, there are some source-code analyzers, who try to do the same.

But in the end it comes back to the programmer knowing what he is doing.

Aniket Salunkhe wrote:
3. What is difference between a MACRO variable & const variable in C/C++? As we can pass or return const variables to/from functions, Can we do same with MACRO variable?

A Macro is something like a search & replace you can do with the text-editor. It is done by the preprocessor before the compiler runs.
The compiler only ever gets to see the de-MACROed source. So, it has no notion whatsoever about MACROS.
So, the answer your specific question is:
No, you can not pass or return a MACRO. But we might devise a MACRO that does not give a compiler error when written instead of a parameter or return type.

Aniket Salunkhe wrote:
4. Is the 'Data Abstraction' is feature of C++?

I have no idea where you got that term from.
after reading wikipedia[^], I think you maybe mean abstract base classes?

Aniket Salunkhe wrote:
Then what do u mean by 'Data Abstraction'? Creating new data types? But we can create new data types in C using struct/union, then why 'Data Abstraction' is not feature of C?

Because C does not explicitly have it as a feature?
Remenber, whil C++ can be expressed as C, C in turn can be expressed as machine language. Yet no one will call machine language abstract.

Let's think the unthinkable, let's do the undoable, let's prepare to grapple with the ineffable itself, and see if we may not eff it after all.
Douglas Adams, "Dirk Gently's Holistic Detective Agency"

Andy Rama12-Feb-08 22:36

12-Feb-08 22:36

Thanks for the reply Rose | [Rose]

Matthew Faithfull12-Feb-08 2:40

Matthew Faithfull

12-Feb-08 2:40

Hi, I have some answers perhaps.

1. You would override the global new and delete operatos if yuo wanted to use your own heap memory manager rather than the one in the C Runtime for all memory management in your process. C# does this to implement 'managed memory' for you.

You would override the new and delete operators within a class if you wanted to implement some special allocation scheme for instances of that class or to selectively make it use your own memory manager.

2. Log every allocation with a unique number and every deallocation with a matching number. One cancels the other, anything left at the end is a leak. Sounds simple but not easy in practice.

3. MACROs dont declare variables they do the equivalent of #define wordy SomethingElse. Unless SomethingElse is also validly decalred you can't use wordy as a variable.

4. What C++ can do that C can't is 'data hiding'. Declaring private or protected members in a class anf=d then giving access to them only through, for example, Get and Set functions. This allows you to error check and trace all accesses and potentially to change the underlying name or even type of the member without breaking any other code as long as the accessors still work and have the same signatures everything is Cool | :cool:

5. Yes, just remember to use the matching version of free.

6. global variables are global, enough said. Class statics are global but their names are class specific, so CAButton::s_MyStaticMember is refered to as such and does not clash with CAScroller::s_MyStaticMember which is a compleely different variable. C++ initializes statics using some Compiler/Linker trickery and bits bootstrap code in the CRuntime which can be a bit messy.

Nothing is exactly what it seems but everything with seems can be unpicked.

Andy Rama12-Feb-08 21:54

12-Feb-08 21:54

3. Is there any difference in #define MAX 30 & const int max=30;?
5. How to use free in that case?

Matthew Faithfull12-Feb-08 22:26

Matthew Faithfull

12-Feb-08 22:26

Aniket Salunkhe wrote:
3. Is there any difference in #define MAX 30 & const int max=30;?

Yes. #define MAX 30 just replaces each occurance of MAX in the code with 30 before it's compiled, const int max=30; declares a variable which will exist at runtime. In the first case int* pInt = &MAX; would give a compiler error in the second int* pInt = &max would be fine.

Aniket Salunkhe wrote:
5. How to use free in that case?

I can't honestly remember, it's years since I used calloc in anger. You'll need to either look at the docs or in the MS C Runtime source which ships as an optional install component with Visual Studio. Every memory allocation function e.g. malloc, calloc, has a specific matching from of free and it's especially important to match them up in Debug builds where they get mapped to diagnostic versions of the functions otherwise you'll get spurious leak reports and heap damaged block reports from the allocation tracking that is added to the debug functions. The CRT memory functions are a tangled multilayer mess, especially in Debug builds and it's not easy to track down the causes of problems after they've occured.

Nothing is exactly what it seems but everything with seems can be unpicked.

Andy Rama12-Feb-08 22:32

12-Feb-08 22:32

Thank You Very Much Rose | [Rose]

set left and right

gentleguy11-Feb-08 23:26

11-Feb-08 23:26

dear all

after i followed the book what it explained us how to use set left and right, but however after i set it, still problem. anyone can tell me detail, thanks

#include <iomanip>
using std::left;
using std::right;
using std::cout;
using std::endl;

for example: cout <<left<<name<<endl;

Li Zhiyuan

CPallini11-Feb-08 23:37

11-Feb-08 23:37

What is your problem?

If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler.
-- Alfonso the Wise, 13th Century King of Castile.

This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong.
-- Iain Clarke

gentleguy12-Feb-08 0:15

12-Feb-08 0:15

after i set left,output value can't be lefted by one column

Li Zhiyuan

CPallini12-Feb-08 0:26

12-Feb-08 0:26

Not sure I've understood. Could you please post an example?

gentleguy12-Feb-08 1:04

12-Feb-08 1:04

for example:

cout << setw(3)<<kk<<left<<setprecision(2)<<name<<endl;

but the output is still as follows:

1 jok
2 nice
.
.
.
10 mike

actually here name mike has same line with nice, jok..but mike is further front...

another question is setw(3), also couldnot work properly..no space display

Li Zhiyuan

CPallini12-Feb-08 2:09

12-Feb-08 2:09

If I understand your requirements then you probably need:

cout << right << setw(3)<< kk << " " << left << name << endl;

gentleguy12-Feb-08 19:45

12-Feb-08 19:45

thanks, i did like this before, could you provide me with detail step by step, thanks a lot. in vc++ 2005 format.

Li Zhiyuan

CPallini12-Feb-08 21:30