Question

I have no idea why this code complies :

int array[100];
array[-50] = 100; // Crash!!

…the compiler still compiles properly, without compiling errors, and warnings.

So why does it compile at all?

Answer 1

array[-50] = 100;

Actually means here:

*(array - 50) = 100;

Take into consideration this code:

int array[100];
int *b = &(a[50]);
b[-20] = 5;

This code is valid and won’t crash. Compiler has no way of knowing, whether the code will crash or not and what programmer wanted to do with the array. So it does not complain.

Finally, take into consideration, that you should not rely on compiler warnings while finding bugs in your code. Compilers will not find most of your bugs, they barely try to make some hints for you to ease the bugfixing process (sometimes they even may be mistaken and point out, that valid code is buggy). Also, the standard actually never requires the compiler to emit warning, so these are only an act of good will of compiler implementers.