Why do online compilers e.g Programiz sometimes get different outputs from code editors like Visual Studio Code?

Question

I was trying to compile this and I was shocked. Online gdb/Programiz/Paiza.io's outputs were all hlpmme, not hlp me. This app and Visual Studio ran the code well, so I really do not understand what happened.
char s[300]="help me";
strcpy(s+1, s+2);
cout<<s;

I also had a similar issue a long time ago when I was working with floats.

Answer 1

Brian There is no "correct" output in this scenario, since calling strcpy() with overlapping strings is undefined behaviour. Also, s + 1 is not the same as s[1], since the resulting pointer is not dereferenced.

Answer 2

Thanks Ipang. Shadow thanks for your reply. Now I understand that not all implementations use safe memory moves that properly handle overlapped memory regions. I think you happened to see my reply about s[1] before I edited it to add the pointer reference. Nonetheless, thank you for pointing that out, too.

I had begun typing up an explanation, but I think the Stack Overflow link covers it better.

Answer 3

Shadow is correct 👍

https://stackoverflow.com/questions/27546706/result-of-calling-strcpy-is-different-than-expected

Answer 4

What does adding an integer to a char array mean in C++?

Answer 5

Denis as you expected, the correct output from any compiler and string library should be "hlp me". It seems that Programiz has a bizarre version of strcpy that skips past whitespace. I have never seen that before. It is perplexing to imagine why that would be useful.

Lisa s+1 is doing pointer math. It is the same as &(s[1]).

Answer 6

Its not cause of the compiler... U might even get different result on same compliler in thi situation