how can i use char * variable in cin ?

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I wrote this code:

char * str1="hello world!";
cin>>str1;
cout<<endl<<str1;

But, why does it not work?

Posted 23-Jun-11 2:21am

Reza Oruji

Updated 23-Jun-11 4:17am

fjdiewornncalwe

v2

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Chris Meech 23-Jun-11 8:23am

It would be helpful if you could explain what "doesn't work!" means. Do you have compiler errors? Linker errors. When it runs, what do you expect for results?

Reza Oruji 23-Jun-11 8:27am

no there is no compiler errors,but it doesn't print the value that i input

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OriginalGriff · Accepted Answer · 2011-06-23T02:26:00

Solution 1

It doesn't work because str1 is pointing to a Constant string: I.e a read-only string. You cannot modify it!

cin is an input stream - you read from it into teh variable.
cout is an output stream - your write to it from a variable.

If you want to read a string from the user, try this:

char[100] text;
cin >> text;
cout << text << endl;

char * str1="hello world!";
cout<<endl<<str1;
str1 = "hello";
cout<<endl<<str1;
- str1 now is "Hello"

Yes - because you have moved the pointer to a different area of memory.
Instead of referring to a constant string "hello world!" terminated by a null character, it now points to a different constant string "Hello" (terminated by a null character). The original contents of the string are unchanged, you are just pointing at different memory (and you still can't overwrite it!)
Try this:

char* str1="hello world!";
char* str2 = str1;
cout<<endl<<str1;
str1 = "hello";
cout<<endl<<str1;
cout<<endl<<str2;

"I did not try to overwrite it. I just got wrong the line "you can not modify it". More interesting why compiler can not generate any error in this case."

Yes you did! :laugh:

char * str1="hello world!";
cin>>str1;

The cin tries to write whatever the user types into the memory pointed at by str1. This is an write operation, thus an attempt to overwrite the string content.
The compiler will not generate an error - it can't - because there is no way for it to know (in all cases) that the memory is read only:

char * str1="hello world!";
char[100] str2;
GetString(str2);
GetString(str1);
...
void GetString(char* pstr)
   {
   cin >> pstr;
   }

The first call to GetString is fine, the second is a run time error.

Posted 23-Jun-11 2:26am

OriginalGriff

Updated 23-Jun-11 4:45am

v3

Comments

Sergey Chepurin 23-Jun-11 9:57am

char * str1="hello world!";
cout<<endl<<str1;

str1 = "hello";

cout<<endl<<str1;
- str1 now is "Hello

OriginalGriff 23-Jun-11 10:10am

Answer updated.

fjdiewornncalwe 23-Jun-11 10:19am

Excellent description of what is going on with the pointer. +5

Sergey Chepurin 23-Jun-11 10:37am

I did not try to overwrite it. I just got wrong the line "you can not modify it". More interesting why compiler can not generate any error in this case.

OriginalGriff 23-Jun-11 10:45am

Answer updated

Sergey Alexandrovich Kryukov 23-Jun-11 14:03pm

I guess, long-standing tradition of C/C++ sloppiness...
--SA

OriginalGriff 23-Jun-11 14:13pm

:laugh:
It's not obvious to a beginner, I guess. (Or even obvious why "Hello World!" should be read only in the first place!)

Sergey Alexandrovich Kryukov 24-Jun-11 19:16pm

:-)

Sergey Chepurin 23-Jun-11 10:58am

I'am not a compiler expert, but it seems clear that at compile time str1 points to constant string (thus, can not be written->error) and str2 is just uninitialized array (can be written->fine). I mean, it is absolutely obvious in initial sample (without GetString), and from GetString() implementation seems clear at compile time that cin>> will try to overwrite the constant string str1 passed to it.
Update:
I found the comment concerning this issue (http://stackoverflow.com/questions/5582552/i-cant-understand-why-segmentation-fault-error-occurred-in-c):
"...string literals are (regular) arrays of characters with the extra caveat of attempts to change the characters result in undefined behaviour. An attempt to change a constant element MUST BE diagnosed by the compiler. – pmg"

OriginalGriff 23-Jun-11 14:11pm

Yes it is obvious to you and me; but the compiler must be coded to detect it: what if the situation was more complex? Would you still expect the compiler to issue an error that it is a possibility that it might be read only memory? Or would that just be frustrating when you try to code logic that means the event can in practice not occur but in theory could? The compiler writers take the safe option: Don't complain and let the run time catch it - that way it is consistent and you only fall for it once! :laugh:

Sergey Alexandrovich Kryukov 23-Jun-11 14:02pm

My 5. I was afraid that nobody would help OP in such a great trouble. :-)
--SA

Sergey Chepurin 23-Jun-11 14:33pm

As no one in stackoverflow.com you did not comment on this: "An attempt to change a constant element MUST BE diagnosed by the compiler".
Update - and here is the answer:http://www.glenmccl.com/ansi_014.htm

CPallini · Accepted Answer · 2011-06-23T02:33:00

It cannot work, because cin is trying to write to a read-only memory area (str1 points to a string literal). You can verify that the following code (pease use short inputs!)

char str1[] = "hello world!";
cin>>str1;
cout<<endl<<str1;

will instead work.

However this is really poor C++ programming: use std::string instead of C-like array of characters, for your strings. e.g.

C++

#include <iostream>
#include <string>
using namespace std;

int main()
{
 string str1 = "hello world!";
 cin >> str1;
 cout << endl << str1;
}

Harmanjeet Singh · Accepted Answer · 2011-06-23T03:13:00

CSS

char * str1="hello world!";
cin>>str1;
cout<<endl<<str1;

This is the code that you are asking for

let me go line by line

char * str1="hello world!";

in the above statement, str1 is a character pointer which points to the string "hello world!"
this is ok...

now you did
cin>>str1;

this is where you are asking for input from the user
which causes it to force the pointer to act as character variable buffer, which is wrong

ptr1 is a pointer, not a buffer in which you can store your string
moreover as ptr1 is pointer, it can only store addresses not buffer again...

so thats the mistake..

the correct code is..

char* str1 = "hello world!";
// cin>>str1; commented this line as its creating the trouble
cout<<endl<<str1;

====================================================

the other way, if you are asking for input from the user, do it like this...
//just modifying your code a little bit
char[] str1 = "hello world"; //using an array instead of pointer
cin>>str1;
cout<<endl<<str1;

how can i use char * variable in cin ?

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