C / C++ / MFC

Pass by pointer not working

17-May-16 20:50

Member 12529159 wrote:
There are, I think, some big mistakes

Yes, this will not compile, and even if it did, none of this code is going to work. You are reading a string into the array seq and then using Push to copy characters back into the same array. You are also storing everything as characters but when you use Pop, you think you are rectrieving an integer. And you assume that each string will be two digits followed by a mathematical symbol.

Joe Smith IX15-May-16 5:52

Joe Smith IX

15-May-16 5:52

Hi all, I have a variable declared:

C++

CMyDlg* m_pDlg1;

I initialized it as NULL and expected it to change when passed to CallDlg. But it stays as NULL before and after the call, therefore creating new modeless CMyDlg everytime it gets called (instead of bringing up the existing one the foreground).

What did I do wrong? Shouldn't m_pDlg1 point to the newly created dialog?
Please help. Thanks in advance.

C++

void CMyApp::ActivateDlg1() 
{ 
  CallDlg(1, m_pDlg1); // the variable stays as NULL
}

void CMyApp::CallDlg(int nType, CMyDlg* pDlg)
{
  if(pDlg) pDlg->SetForegroundWindow(); // This never gets called
  else
  {
    pDlg = new CMyDlg(nType);
    if(!::IsWindow(pDlg->GetSafeHwnd())) pDlg->Create(IDD_MY_DLG, NULL);
  }
}

Victor Nijegorodov15-May-16 10:58

Victor Nijegorodov

15-May-16 10:58

Joe Smith IX wrote:
I initialized it as NULL and expected it to change when passed to CallDlg. But it stays as NULL before and after the call, therefore creating new modeless CMyDlg everytime it gets called (instead of bringing up the existing one the foreground).

1. I do not see where, how and when you set it to NULL.
2. Did you debug your code to see the "real" values of all the variables?
3. To make debugging easier for yourself you should avoid write the code in the manner

if(pDlg) pDlg->SetForegroundWindow();

but

if(pDlg)
    pDlg->SetForegroundWindow();

4. how and where do you delete the m_pDlg1 pointer?

Re: Pass by pointer not working [fixed]

Joe Smith IX17-May-16 8:34

Joe Smith IX

17-May-16 8:34

1. I didn't copy the whole code, just the snippet.
2. Yes, I did. That's how I could state 'it stays as NULL before and after the call'.
3. I do. Thanks.
4. Same reason as number 1 above.

Anyway, I solved it as pointed by CPallini below. I was missing the ampersand symbol to indicate passing by pointer. Thanks anyway.

CPallini15-May-16 22:08

CPallini

15-May-16 22:08

If you want to change the passed pointer value then you have to pass its address (because, like any other kind of parameter, pointers are passed by value) ending up with a double pointer or a reference to the pointer. e.g.

C++

void CMyApp::CallDlg(int nType, CMyDlg & * pDlg)
void CMyApp::CallDlg(int nType, CMyDlg * & pDlg)
{
 //...
}

modified 17-May-16 15:57pm.

Joe Smith IX17-May-16 8:25

Joe Smith IX

17-May-16 8:25

Ah, you are right. To pass by address, I need the & character. It's working fine now. Thanks so much.

void CMyApp::CallDlg(int nType, CMyDlg* &pDlg) 
{
  // ... 
}

CPallini17-May-16 9:58

CPallini

17-May-16 9:58

I gave you a flawed code sample, sorry. Blush | :O

Thank you for pointing it out so nicely.

C string operations

Member 1096409913-May-16 5:47

Member 10964099

13-May-16 5:47

if I have this code :
char * s =(char*)malloc(10);
and then I write 's="test"' then s will have a different address right ?
why is this happening ?

Richard MacCutchan13-May-16 6:10

13-May-16 6:10

You are ~~allocating~~ assigning the address of the string literal "test" to the pointer s thus overwriting the address returned by malloc. You should be using strcpy to copy the four characters into the allocated buffer.

[edit]
better terminology as pointed out by member below.
[/edit]

modified 17-Jun-16 4:11am.

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C^hris L^osinger13-May-16 6:37

C^hris L^osinger

13-May-16 6:37

or, even better, strncpy.

Richard MacCutchan13-May-16 6:43

13-May-16 6:43

Absolutely right, but I was trying to KISS.

Member 1256233616-Jun-16 12:40

Member 12562336

16-Jun-16 12:40

Shouldn't that be: "you are assigning the address" instead of allocating?

Richard MacCutchan16-Jun-16 22:10

16-Jun-16 22:10

Of course.

Patrice T14-May-16 3:05

Patrice T

14-May-16 3:05

Member 10964099 wrote:
why is this happening ?

It append because it designed this way. That is how pointers work.

I recommend to read THE book "the C Language" from Kernigan & Ritchie.
The C Programming Language - Wikipedia, the free encyclopedia[^]

Patrice

“Everything should be made as simple as possible, but no simpler.” Albert Einstein

CPallini14-May-16 3:06

CPallini

14-May-16 3:06

Because s is a pointer.

char * s =(char*)malloc(10); // here s points to the freshly allocated memory buffer

s="test" // here s points to the memory containing the string literal

Re: C execl is this a bug ?

leon de boer14-May-16 5:58

leon de boer

14-May-16 5:58

What all the others said and more generally rarely if ever do use an equal sign on a string in C at any other time than creation.

There are functions required to add, copy, compare, split and search strings and you can't just use mathematical symbols like "=" and "+" to do things with strings like you can in some higher languages. Strings are not mathematical types and you can't use mathematical functions to do things with them.

Those coming from a coding background in Java or Basic tend to do what you did and you have to unlearn it when using C because "string1" + "string2" will not work either, you will need to use strcat functions. String compares are likewise a function and you can't use mathematical equality signs to compare two strings.

Learning the string functions, it is a requirement of learning the C language.

In vino veritas

modified 14-May-16 12:10pm.

C execl is this a bug ?

Member 1096409913-May-16 5:37

Member 10964099

13-May-16 5:37

inside a C function I have this code

printf("%s",final);
//execl(final,final,NULL);

I do a fork and this is part of the childs code
the final string is not printed when the execl line is not commented out,nor are any printf calls before it
why is this happening ?

Richard MacCutchan13-May-16 6:07