Parentheses that change everything

It is well known — and intuitively understood by most — that adding a set of parentheses usually doesn’t change anything; for example, int answer = 42; is equal to int answer = (42); or int answer = ((42));. There are some important exceptions to that rule, however, and I’ll talk about these in this post.


Although macros are rarely used in good C++ code, it is important to be able to understand what’s happening and why. Using a popular example of MIN macro, the naïve implementation would look like this:

#define MIN(x,y) x < y ? x : y

To a beginner, this would look like a correct implementation, and indeed, it would work in some cases; for example, answer below would indeed be equal to 42:

int answer = MIN(42,50);

Unfortunately, macros are expanded as text, and in the following example, possibly surprisingly, answer would hold the value of 41 instead:

int answer = 2 + MIN(40,41);