Inadvertent use of = instead of ==

Question

It seems that

if (x=y) { .... }

instead of

if (x==y) { ... } 

is a root of many evils.

Why don't all compilers mark it as error instead of a configurable warning?

I'm interested in finding out cases where the construct if (x=y) is useful.

Answer 1

Most of the time, compilers try very hard to remain backward compatible.

Changing their behavior in this matter to throw errors will break existing legitimate code, and even starting to throw warnings about it will cause problems with automatic systems that keep track of code by automatically compiling it and checking for errors and warnings.

This is an evil we're pretty much stuck with at the moment, but there are ways to circumvent and reduce the dangers of it.

Example:

   void *ptr = calloc(1, sizeof(array));
   if (NULL = ptr) {
       // Some error
   }

This causes a compilation error.