How do you reverse a string in place in C or C++?

Answer

#include<stdio.h>void strrev(char*p){char*q = p;while(q &&*q)++q;for(--q; p < q;++p,--q)*p =*p ^*q,*q =*p ^*q,*p =*p ^*q;}int main(int argc,char**argv){do{
    printf("%s ",  argv[argc-1]);
    strrev(argv[argc-1]);
    printf("%s\n", argv[argc-1]);}while(--argc);return0;}

(This is XOR-swap thing. Take care to note that you must avoid swapping with self, because a^a==0.)

Ok, fine, let's fix the UTF-8 chars...

#include<bits/types.h>#include<stdio.h>#define SWP(x,y)(x^=y, y^=x, x^=y)void strrev(char*p){char*q = p;while(q &&*q)++q;/* find eos */for(--q; p < q;++p,--q) SWP(*p,*q);}void strrev_utf8(char*p){char*q = p;
  strrev(p);/* call base case *//* Ok, now fix bass-ackwards UTF chars. */while(q &&*q)++q;/* find eos */while(p <--q)switch((*q &0xF0)>>4){case0xF:/* U+010000-U+10FFFF: four bytes. */
      SWP(*(q-0),*(q-3));
      SWP(*(q-1),*(q-2));
      q -=3;break;case0xE:/* U+000800-U+00FFFF: three bytes. */
      SWP(*(q-0),*(q-2));
      q -=2;break;case0xC:/* fall-through */case0xD:/* U+000080-U+0007FF: two bytes. */
      SWP(*(q-0),*(q-1));
      q--;break;}}int main(int argc,char**argv){do{
    printf("%s ",  argv[argc-1]);
    strrev_utf8(argv[argc-1]);
    printf("%s\n", argv[argc-1]);}while(--argc);return0;}

• Why, yes, if the input is borked, this will cheerfully swap outside the place.
• Useful link when vandalising in the UNICODE: http://www.macchiato.com/unicode/chart/
• Also, UTF-8 over 0x10000 is untested (as I don't seem to have any font for it, nor the patience to use a hexeditor)

Examples:

$ ./strrev Räksmörgås ░▒▓○◔◑◕●░▒▓○◔◑◕●●◕◑◔○▓▒░

Räksmörgås sågrömskäR

./strrev verrts/.