Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.c++ > #80660 > unrolled thread

Trying to understand pointers. Why does this give unexpected results?

Back to article view | Back to comp.lang.c++

Contents

  Trying to understand pointers.  Why does this give unexpected results? rob <rob@drrob.com> - 2021-07-03 16:12 -0400
    Re: Trying to understand pointers.  Why does this give unexpected results? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-07-03 21:27 +0100
      Re: Trying to understand pointers.  Why does this give unexpected results? rob <rob@drrob.com> - 2021-07-03 17:54 -0400
        Re: Trying to understand pointers. Why does this give unexpected results? Real Troll <real.troll@trolls.com> - 2021-07-03 22:04 +0000
          Re: Trying to understand pointers. Why does this give unexpected results? "Fred. Zwarts" <F.Zwarts@KVI.nl> - 2021-07-15 09:45 +0200
    Re: Trying to understand pointers.  Why does this give unexpected results? Juha Nieminen <nospam@thanks.invalid> - 2021-07-08 08:17 +0000

#80660 — Trying to understand pointers. Why does this give unexpected results?

This is my test program

#include everything relevant here
int main {
  int a = 0;
  float b = 1.0;
  char c = 'c';

  int *ptrA;
  float *ptrB;
  char *ptrC;

  ptrA = &a;
  ptrB = &b;
  ptrC = &c;

  cout << "value of a: " << a << "; address of a: "  << ptrA << endl;
  cout << "vaue of b: " << b << "; address of b: " << ptrB << endl;
  cout << "value of c: " << c << "; address of c: " << ptrC << endl;

  return 0;
}


This program, compiled w/ gcc 9.3 on Pop_OS 20.04, does not print an
address for c.  It just prints "c"; this does show addresses for a and
b.

Why can't I show the address for the char variable c?

Thx,
Rob

[toc] | [next] | [standalone]

#80661

rob <rob@drrob.com> writes:

> This is my test program
>
> #include everything relevant here
> int main {

And you need ()s there and at least a "using namespace std;" unless you
are using a museum version of C++.

>   int a = 0;
>   float b = 1.0;
>   char c = 'c';
>
>   int *ptrA;
>   float *ptrB;
>   char *ptrC;
>
>   ptrA = &a;
>   ptrB = &b;
>   ptrC = &c;
>
>   cout << "value of a: " << a << "; address of a: "  << ptrA << endl;
>   cout << "vaue of b: " << b << "; address of b: " << ptrB << endl;
>   cout << "value of c: " << c << "; address of c: " << ptrC << endl;

What would you like this code to do:

   char *string = "Hello world\n";
   cout << string;

?  Can you see the problem now?

>   return 0;
> }
>
>
> This program, compiled w/ gcc 9.3 on Pop_OS 20.04, does not print an
> address for c.  It just prints "c"; this does show addresses for a and
> b.
>
> Why can't I show the address for the char variable c?

You can if you do this:

  cout << "value of c: " << c << "; address of c: " << (void *)ptrC << endl;

-- 
Ben.

[toc] | [prev] | [next] | [standalone]

#80663

Thanks, that helps me.




>> This program, compiled w/ gcc 9.3 on Pop_OS 20.04, does not print an
>> address for c.  It just prints "c"; this does show addresses for a and
>> b.
>>
>> Why can't I show the address for the char variable c?
>
>You can if you do this:
>
>  cout << "value of c: " << c << "; address of c: " << (void *)ptrC << endl;

[toc] | [prev] | [next] | [standalone]

#80664 — Re: Trying to understand pointers. Why does this give unexpected results?

On 03/07/2021 22:54, rob wrote:
> Thanks, that helps me.
>
>
>
>
>>> This program, compiled w/ gcc 9.3 on Pop_OS 20.04, does not print an
>>> address for c.  It just prints "c"; this does show addresses for a and
>>> b.
>>>
>>> Why can't I show the address for the char variable c?
>> You can if you do this:
>>
>>   cout << "value of c: " << c << "; address of c: " << (void *)ptrC << endl;

You could also do something like this:

> #include <iostream>
>
> using namespace std;
>
> int main()
> {
>   int a = 0;
>   float b = 1.0;
>   char c[] = "c";
>
>   int *ptrA;
>   float *ptrB;
>   char *ptrC;
>
>   ptrA = &a;
>   ptrB = &b;
>   ptrC = c;
>
>   cout << "value of a: " << a << "; address of a: " << ptrA << endl;
>   cout << "vaue of b: " << b << "; address of b: " << ptrB << endl;
>   cout << "value of c: " << c << "; address of c: " << &ptrC << endl;
>
>   return 0;
> }

[toc] | [prev] | [next] | [standalone]

#80712 — Re: Trying to understand pointers. Why does this give unexpected results?

Op 04.jul..2021 om 00:04 schreef Real Troll:
> On 03/07/2021 22:54, rob wrote:
>> Thanks, that helps me.
>>
>>
>>
>>
>>>> This program, compiled w/ gcc 9.3 on Pop_OS 20.04, does not print an
>>>> address for c.  It just prints "c"; this does show addresses for a and
>>>> b.
>>>>
>>>> Why can't I show the address for the char variable c?
>>> You can if you do this:
>>>
>>>    cout << "value of c: " << c << "; address of c: " << (void *)ptrC << endl;
> 
> You could also do something like this:
> 
>> #include <iostream>
>>
>> using namespace std;
>>
>> int main()
>> {
>>    int a = 0;
>>    float b = 1.0;
>>    char c[] = "c";
>>
>>    int *ptrA;
>>    float *ptrB;
>>    char *ptrC;
>>
>>    ptrA = &a;
>>    ptrB = &b;
>>    ptrC = c;
>>
>>    cout << "value of a: " << a << "; address of a: " << ptrA << endl;
>>    cout << "vaue of b: " << b << "; address of b: " << ptrB << endl;
>>    cout << "value of c: " << c << "; address of c: " << &ptrC << endl;

Doesn't that print the address of ptrC, instead of the address of c?

>>
>>    return 0;
>> }
> 
> 
> 

[toc] | [prev] | [next] | [standalone]

#80691

rob <rob@drrob.com> wrote:
>   int *ptrA;
>   float *ptrB;
>   char *ptrC;
> 
>   ptrA = &a;
>   ptrB = &b;
>   ptrC = &c;

I'm genuinely wondering why you are writing it like that, instead of the
simpler:

  int *ptrA = &a;
  float *ptrB = &b;
  char *ptrC = &c;

>   cout << "value of c: " << c << "; address of c: " << ptrC << endl;

A char* pointer is overloaded to print the string pointed to by that pointer,
so that will severely malfunction. You can cast it to void* instead:

  std::cout << static_cast<void*>(ptrC) << std::endl;

[toc] | [prev] | [standalone]

Back to top | Article view | comp.lang.c++

csiph-web