do while(false) pattern [duplicate]

Answer 1

People use it because otherwise, you can screw up your ifs with compound statements. Imagine

#define hai int x; \
x = 0;

if (condition)
    hai;
else
    func();

Imagine what the preprocessed source looks like.

if (condition)
   int x;
   x = 0;
else
   func();

Oh wait- now our else doesn't work.

Macros like that however are typically unnecessary in C++.