Conditional directives

Conditional Directives

Function: tpp.c TPPLexer_Next
Function: tpp.c TPPLexer_EvalConst
Function: tpp.c TPPLexer_EvalConst1

The c11 standard defines a variety of directives, to control what code is visible to the compiler after preprocessing is done. TPP implements all of them with only minor additions:

1. #if
2. #elif
3. #else
4. #endif
5. #ifdef
6. #ifndef

Conditional expressions that follow #if and #elif follow the obvious rules and allow the use of all of the c operators. The internal value used for evaluating the expression is usually a 64-bit integer (but no guaranties are given on that).

Supported operators are (in precedence order):
1. , (comma operator: only available, when evaluating using TPPLexer_EvalConst)
2. ? (Conditional operator)
3. || (Logical or)
4. ^^ (Extension: Logical xor)
5. && (Logical and)
6. | (Or)
7. ^ (Xor)
8. & (And)
9. == (Equal) != (Not equal)
10. < (Lower than) <= (Lower equal) > (Greater than) >= (Greater equal)
11. << (Shift left) >> (Shift right)
12. + (Plus) - (Minus)
13. * (Multiply) / (Divide) % (Modulo)
14. + (Unary plus) - (Unary minus) ! (Logical not) ~ (Complement) <int> (Unary constant) (...) (Parenthesis)

Check: logical xor: #if __has_feature(tpp_token_lxor) || __has_extension(tpp_token_lxor)

Directives may not cross multiple files and a warning is emitted, if not all #if-blocks are closed at the end of a file.

The following example shows the different directives working together:

#if 10+20 == 30
// Enabled
#elif 1
// Disabled
#else
// Disabled
#endif

#if 49*10 == 100
// Disabled
#elif 42 != 42
// Disabled
#elif 42 == 42
// Enabled
#else
// Disabled
#endif

#define foobar

#if defined(foobar)
// Enabled
#endif

#if !defined foobar
// Disabled
#elif defined foobar
// Enabled
#endif

#ifdef foobar
// Enabled
#endif

#ifndef foobar
// Disabled
#endif

#if 1
#ifdef foobar
#ifndef foobar2
// Enabled
#endif
#endif
#endif