Cpp Memo part 5 -- Conditionals

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Cpp Memo part 5 – Conditionals

1 Prewords

A "conditional" is a directive that instructs the preprocessor to select whether or not to include a chunk of code in the final token stream passed to the compiler. Preprocessor conditionals can:

  • Test arithmetic expressions, or
  • Test whether a name is defined as a macro, or
  • both simultaneously using the special `defined' operator.

A conditional in the C preprocessor resembles in some ways an `if' statement in C, but it is important to understand the difference between them.

  • If statement:

    The condition in an `if' statement is tested during the execution of your program. Its purpose is to allow your program to behave differently from run to run, depending on the data it is operating on.

  • If directive

    The condition in a preprocessing conditional directive is tested when your program is compiled. Its purpose is to allow different code to be included in the program depending on the situation at the time of compilation.

2 Conditional Uses

There are three general reasons to use a conditional.

  • A program may need to use different code depending on the machine or operating system it is to run on.

    In some cases the code for one operating system may be erroneous on another operating system; for example, it might refer to data types or constants that do not exist on the other system. When this happens, it is not enough to avoid executing the invalid code. Its mere presence will cause the compiler to reject the program. With a preprocessing conditional, the offending code can be effectively excised from the program when it is not valid.

  • You may want to be able to compile the same source file into two different programs.

    One version might make frequent time-consuming consistency checks on its intermediate data, or print the values of those data for debugging, and the other not.

  • A conditional whose condition is always false is one way to exclude code from the program but keep it as a sort of comment for future reference.

Simple programs that do not need system-specific logic or complex debugging hooks generally will not need to use preprocessing conditionals.

3 Conditional Syntax

A conditional in the C preprocessor begins with a "conditional directive": `#if', `#ifdef' or `#ifndef'.

3.1 Ifdef

The simplest sort of conditional is

#ifdef MACRO

CONTROLLED TEXT

#endif /* MACRO */

This block is called a "conditional group". CONTROLLED TEXT will be included in the output of the preprocessor if and only if MACRO is defined. We say that the conditional "succeeds" if MACRO is defined, "fails" if it is not.

Sometimes you wish to use some code if a macro is not defined. You can do this by writing `#ifndef' instead of `#ifdef'. One common use of `#ifndef' is to include code only the first time a header file is included.

#ifndef _TEST_H_
#define _TEST_H_

// #ifdef __cplusplus
// extern "C" {
// #endif


// #ifdef __cplusplus
// }
// #endif

#endif /* _TEST_H_ */

3.2 If

The `#if' directive allows you to test the value of an arithmetic expression, rather than the mere existence of one macro. Its syntax is

#if EXPRESSION

CONTROLLED TEXT

#endif /* EXPRESSION */

EXPRESSION is a C expression of integer type, subject to stringent restrictions. It may contain

  • Integer constants.
  • Character constants, which are interpreted as they would be in normal code.
  • Arithmetic operators for addition, subtraction, multiplication, division, bitwise operations, shifts, comparisons, and logical operations (`&&' and `||'). The latter two obey the usual short-circuiting rules of standard C.
  • Macros. All macros in the expression are expanded before actual computation of the expression's value begins.
  • Uses of the `defined' operator, which lets you check whether macros are defined in the middle of an `#if'.
  • Identifiers that are not macros, which are all considered to be the number zero. This allows you to write `#if MACRO' instead of `#ifdef MACRO', if you know that MACRO, when defined, will always have a nonzero value. Function-like macros used without their function call parentheses are also treated as zero.

    In some contexts this shortcut is undesirable. The `-Wundef' option causes GCC to warn whenever it encounters an identifier which is not a macro in an `#if'.

The preprocessor does not know anything about types in the language. Therefore, `sizeof' operators are not recognized in `#if', and neither are `enum' constants. They will be taken as identifiers which are not macros, and replaced by zero. In the case of `sizeof', this is likely to cause the expression to be invalid.

3.3 Defined

The special operator `defined' is used in `#if' and `#elif' expressions to test whether a certain name is defined as a macro. `defined NAME' and `defined (NAME)' are both expressions whose value is 1 if NAME is defined as a macro at the current point in the program, and 0 otherwise. Thus, `#if defined MACRO' is precisely equivalent to `#ifdef MACRO'.

`defined' is useful when you wish to test more than one macro for existence at once. For example,

#if defined (__vax__) || defined (__ns16000__)

would succeed if either of the names `__vax__' or `__ns16000__' is defined as a macro.

Conditionals written like this:

#if defined BUFSIZE && BUFSIZE >= 1024

can generally be SIMPLIFIED to just `#if BUFSIZE >= 1024', since if `BUFSIZE' is not defined, it will be interpreted as having the value zero .

3.4 Else

The `#else' directive can be added to a conditional to provide alternative text to be used if the condition fails. This is what it looks like:

#if EXPRESSION
TEXT-IF-TRUE
#else /* Not EXPRESSION */
TEXT-IF-FALSE
#endif /* Not EXPRESSION */

If EXPRESSION is nonzero, the TEXT-IF-TRUE is included and the TEXT-IF-FALSE is skipped. If EXPRESSION is zero, the opposite happens.

You can use `#else' with `#ifdef' and `#ifndef', too.

3.5 Elseif

One common case of nested conditionals is used to check for more than two possible alternatives. For example, you might have

#if X == 1
...
#else /* X != 1 */
#if X == 2
...
#else /* X != 2 */
...
#endif /* X != 2 */
#endif /* X != 1 */

Another conditional directive, `#elif', allows this to be abbreviated as follows:

#if X == 1
...
#elif X == 2
...
#else /* X != 2 and X != 1*/
...
#endif /* X != 2 and X != 1*/

`#elif' stands for "else if". Like `#else', it goes in the middle of a conditional group and subdivides it; it does not require a matching `#endif' of its own. Like `#if', the `#elif' directive includes an expression to be tested. The text following the `#elif' is processed only if the original `#if'-condition failed and the `#elif' condition succeeds.

More than one `#elif' can go in the same conditional group. Then the text after each `#elif' is processed only if the `#elif' condition succeeds after the original `#if' and all previous `#elif' directives within it have failed.

`#else' is allowed after any number of `#elif' directives, but `#elif' may not follow `#else'.

4 Deleted Code

If you replace or delete a part of the program but want to keep the old code around for future reference, you often cannot simply comment it out. Block comments do not nest, so the first comment inside the old code will end the commenting-out. The probable result is a flood of syntax errors.

One way to avoid this problem is to use an always-false conditional instead. For instance, put `#if 0' before the deleted code and `#endif' after it. This works even if the code being turned off contains conditionals, but they must be entire conditionals (balanced `#if' and `#endif').


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