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String literals in C++

Posted on 2004-09-27
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Last Modified: 2008-01-09
The following program seg faults on my machine.

int main() {
    char *cp = "Hell";
    *cp = 'B';
}

while the following works fine.

int main() {
    char cp[] = "Hell";
    *cp = 'B';
}

The following program does not compile.

int main() {
    const char *cp = "Hell";
    *cp = 'B';
}

Can someone explain the three observations?
0
Comment
Question by:priyendra
[X]
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3 Comments
 
LVL 48

Accepted Solution

by:
AlexFM earned 125 total points
ID: 12159234
1) cp points to read-only memory where program string constants are kept. Changing of this memory is access violation.
2) cp points to char aray allocated on stack. It is possible to change this memory.
3) *cp is constant pointer, *cp = ... is not allowed.
0
 

Author Comment

by:priyendra
ID: 12159385
Alex, thanks a lot for your answer. I guess this must be one of the fastest answers I have ever obtained in EE :-) For my other question, I shall await a few more comments before closing.
0
 
LVL 30

Expert Comment

by:Axter
ID: 12162049
FYI:
Not all compilers will put string literals in read-only memory.

Changing the contents of a string literal results in undefined behavior IAW with the C++ standards.

Some compilers will allow you to do this, and others will not.
For example, VC++ 5.0 will allow you to change the contents of a pointer to a string literal.
However, VC++ 6.0/7.x will not.


You should avoid code that results in undefined behavior.
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