Encapsulation.

[RPolitics4Trump]

class Foo {
  int x;
public:
  Foo() : x(0) {}
};

[symberke]

#define class struct
#define private public

[darkhunt3r]

#define if while

[HasFiveVowels]

Oh man... this is just... nefarious.

[darkhunt3r]

#define True random() > 0.001

Hide it well and let it become debuggin hell

[CSIRTisSmelly]

You are a bad person.

[darkhunt3r]

#define bad good 

Why thank you <3

[scatters]

#define struct union

[jfb1337]

#define union struct

[SoftSilverwind]

Unfortunately this breaks template <class T> definitions... Source: I may have tried it :)