Stdbool.h

1. Overview

The <stdbool.h> header file is part of the C Standard Library, introduced in the ISO/IEC 9899:1999 standard (C99).

Prior to C99, C did not have a native boolean data type. Programmers used int to represent logic (where 0 is false and non-zero is true). This header standardizes boolean logic, improving code readability and intent.

2. Core Macro Definitions

The primary function of this header is to define four macros. When preprocessed, they translate as follows:

Macro	Expands To	Description
bool	_Bool	Expands to the intrinsic integer type _Bool introduced in C99.
true	1	Integer constant representing logical truth.
false	0	Integer constant representing logical falsehood.
__bool_true_false_are_defined	1	A signal to the compiler/preprocessor that boolean logic is available.

3. Internal Mechanism

It is important to understand that bool in C is not a new primitive type in the same way int or char are. It is a macro wrapper around _Bool.

• _Bool: This is an unsigned integer type large enough to hold the values 0 and 1.
• Assignment Behavior: If a value is assigned to a _Bool variable, it is converted to 1 if the value is non-zero, and 0 if the value is zero.
  • Example: bool b = 100; results in b storing 1.

4. Code Example

The following example demonstrates standard usage:

#include <stdio.h>
#include <stdbool.h> // Required to use bool, true, false

// Function returning a boolean value
bool is_even(int num) {
    if (num % 2 == 0) {
        return true;  // Returns 1
    } else {
        return false; // Returns 0
    }
}

int main(void) {
    bool flag = true;
    int number = 42;

    if (flag) {
        if (is_even(number)) {
            printf("%d is an even number.\n", number);
        }
    }

    // Demonstrating the underlying integer nature
    printf("Value of true: %d\n", true);   // Output: 1
    printf("Value of false: %d\n", false); // Output: 0

    return 0;
}

5. Historical Context & C++ Compatibility

• C89 / C90 (Legacy):
  • <stdbool.h> does not exist.
  • Developers often manually defined types: typedef int bool; or #define TRUE 1.
• C++:
  • bool, true, and false are keywords in C++. They are built into the language.
  • While you can include <stdbool.h> in C++ for backward compatibility, it is unnecessary and typically deprecated in modern C++ code.

6. Best Practices

1. Use for Readability: Using bool makes function signatures self-documenting (e.g., bool is_valid() is clearer than int is_valid()).
2. Avoid Explicit Comparison:
   • Bad: if (my_bool == true)
   • Good: if (my_bool)
3. Standard Compliance: Always include this header if you use bool in C. Do not rely on compiler extensions that might support it without the include.