string reverse in c++

Question

Why does this piece of code throw an exception when I try to change a character in the string

void reverseString(char *s)
{
    int e = strlen(s) - 1;
    int b = 0;
    char t1,t2;

    while(b < e)
    {
        t1 = s[b];
        t2 = s[e];
        s[b] = t2;
        s[e] = t1;
        b++;
        e--;
    }
}

What exception is being thrown? What line is it thrown from? What is an input to the function that generates this behavior? — abelenky, Oct 05 '10 at 22:27
I figured it out. I was passing in a string, since strings are immutable it was throwing an exception. When I changed it to an array of chars it worked fine. — silencer, Oct 05 '10 at 22:29
This is almost FAQ but you need to show the code that calls you function for it to be answered. — CB Bailey, Oct 05 '10 at 22:30
How did you manage to pass a `std::string`? The compiler will not let you pass it directly, and if you use the `c_str()` method it will complain about it being `const`. Those kinds of warnings, esp. if you have to cast to make it work, should be a red flag. — Nate, Oct 05 '10 at 22:35
@user465353: No code in this function throws any exception whatsoever. Either you are referring to `std::string` which is not immutable, or you are referring to a string literal, which is of type `const char[]`. In both cases, passing them to your function would yield a compilation error, not a runtime exception. — André Caron, Oct 05 '10 at 22:37
possible duplicate of [Why is this C code causing a segmentation fault?](http://stackoverflow.com/questions/1614723/why-is-this-c-code-causing-a-segmentation-fault) — CB Bailey, Oct 05 '10 at 22:40
@André Caron: sadly, the implicit conversion from string literals (const char *) to char * is permitted for (nowadays ridiculous) compatibility reasons with code written when `const` wasn't a part of the C language. Newer compilers will probably emit a warning, but usually nothing more. IIRC, C99 started to deprecate such conversion. — Matteo Italia, Oct 05 '10 at 22:40
when he says "string" i think he means "this is my test string" not a std::string — pm100, Oct 05 '10 at 22:55

score 4 · Accepted Answer · edited Oct 26 '13 at 14:01

4

My guess is that the string you are testing it on is stored in read-only memory. Did you define it with a string literal?

Added later to elaborate:

If you do this,

char *s = "Hello";
reverseString(s);

you will probably crash, because the string can be stored in read-only memory, and most compilers will put it there.

If on the other hand, you write,

char s[] = "Hello";
reverseString(s);

it will work.

edited Oct 26 '13 at 14:01

Martin York

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answered Oct 05 '10 at 22:37

Jive Dadson

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According to a comment that the OP just added, this is indeed the answer. So why the down-vote? Mis-click, one hopes. – Jive Dadson Oct 05 '10 at 22:40
Thank you, this is exactly what I was doing wrong. – silencer Oct 06 '10 at 02:43

score 2 · Answer 2 · answered Oct 05 '10 at 22:37

2

Don't reinvent the wheel, use standard library's algorithms: std::reverse

void reverseString(char *s) {
  std::reverse(s, s + strlen(s));
}

void reverseString(std::string &s) {
  std::reverse(s.begin(), s.end());
}

Note: std::string is mutable.

answered Oct 05 '10 at 22:37

phadej

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I think that he's "reinventing the wheel" for exercise; if not, well, then you have a valid point. :) – Matteo Italia Oct 05 '10 at 22:44
1

@Matteo better exercise is to reinvent the `std::reverse` :) – phadej Oct 05 '10 at 22:57
Well, this is out of discussion (although I think that there isn't much difference from the algorithm point of view). :) – Matteo Italia Oct 05 '10 at 23:52

string reverse in c++

2 Answers2