CODEWITHSUNDEEP
c++c-preprocessor
Gordem
Gordem

Reputation: 115

Falsify true ↔ false

This question has no practical use! I've asked this only because I'm curious!

There is a way in C++ to falsify true as false by writing somewhere #define true false, and then everywhere true in code will considered as false. But I'm seeking for a way to falsify true as false and false as true at the same time:

#define true false
#define false true

This doesn't works, and trying to "save" original true also doesn't:

#define temptrue true
#define true false
#define false temptrue

Do you know any way to do that?

Upvotes: 1

Views: 264

Answers (4)

jxh
jxh

Reputation: 70482

Perhaps something like this?

#define false static_cast<bool>(1)
#define true  static_cast<bool>(0)

Regarding undefined behavior:

Those that say it is undefined are probably referring to the answer to this question: Is it legal to redefine a C++ keyword?

However, if you do not use the standard C++ library, the cited restriction does not apply (kudos to Bathsheba and Martin York).

16.5.4.1 [constraints.overview]
Subclause 16.5.4 describes restrictions on C++ programs that use the facilities of the C++ standard library.
...
16.5.4.3.2 [macro.names] ...
A translation unit shall not #define or #undef names lexically identical to keywords, ...
C++ 2020 draft

Upvotes: 3

Bathsheba
Bathsheba

Reputation: 234835

The behaviour on attempting to #define a C++ keyword is undefined. Don't do it!

It's not quite so pretty, but

static constexpr bool true_ = false;
static constexpr bool false_ = true;

is probably the best you can do.

Upvotes: 2

Const
Const

Reputation: 1356

use constexpr variables rather changing the behavior of true and false.

static constexpr bool TRUE = false;
static constexpr bool FALSE = true;

Upvotes: 1

Konrad Rudolph
Konrad Rudolph

Reputation: 545985

This obviously has no practical use whatsoever and is not valid C++, but the following does the trick:

static constexpr auto fake_true = false;
static constexpr auto fake_false = true;

#define true fake_true
#define false fake_false

Simply using numeric literals (e.g. 1 and 0) might appear simpler but will cause different semantics in situations where the type matters (e.g. overload resolution).

Upvotes: 1

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