Question

define SIZE 12 How will the preprocessor rewrite the following lines? A) price \(=\operatorname{SIZE} … # Assume the following preprocessor directive appears in a program: # define SIZE 12 How will the preprocessor rewrite the following lines? A) price \)=\operatorname{SIZE} *\( unitcost B) cout \)<<\( setw \)(\operatorname{SIZE})<48.7\( C) cout \)<<\operatorname{SIZ} E$

Short answer

Answer: After replacing the SIZE constant with its value (12), the expressions will look like: A) price = 12 * unitcost B) cout << setw(12) < 48.7 C) cout << SIZ E

Step-by-step solution

- Replace SIZE constant in the given expressions

Identify the places in the expressions where SIZE is used and replace it with its value (12). A) price \(= \operatorname{SIZE} *\) unitcost B) cout \(<<\) setw \((\operatorname{SIZE})<48.7\) C) cout \(<<\operatorname{SIZ} E\)

- Rewrite expressions with replaced SIZE constant

Replace SIZE in each expression with 12 and write the updated expressions. A) price \(= 12 *\) unitcost B) cout \(<<\) setw \((12)<48.7\) C) The third expression has no "SIZE" keyword, so it remains unchanged: cout \(<<\operatorname{SIZ} E\)

Key concepts

C++ Macros

In C++, a macro is a fragment of code which has been given a name. Whenever the name is used, it is replaced by the contents of the macro. Macros are defined using the #define preprocessor directive. This is most often used to create constants that can be reused throughout your code or to implement functions-like constructs known as macro functions.

When you define a macro, the preprocessor will automatically replace all occurrences of the macro with its value before the actual compilation begins. For example, if you define #define PI 3.14, every occurrence of PI in your code will be replaced with 3.14 by the preprocessor.

However, it's important to note that macros don't have a scope and are not type-safe. This means they can potentially lead to errors if not used carefully. The scope of a macro continues until it is undefined with #undef or until the end of the file. A macro can also be redefined unless you specify that it shouldn't be, which can lead to unexpected results if not managed properly.

Preprocessor #define

The #define directive is a powerful tool in C++ that instructs the preprocessor to replace all instances of a given identifier in the code with a specified sequence of characters. It can be used to define both object-like macros, which are typically constants, and function-like macros, which are similar to inline functions.

A common use of #define is for declaring constants, such as #define SIZE 12, as seen in our exercise. This allows for more readable and maintainable code, as SIZE can be used in place of the literal 12 throughout the source code, and can be changed in one place if needed.

Benefits of using #define

• Centralizes the definition of constants, which simplifies updates and maintenance.
• Can improve performance by eliminating the need for function calls in the case of function-like macros.
• Ensures consistency throughout the code as the preprocessor replaces all occurrences.

C++ Constant Expressions

A constant expression in C++ is an expression that consists entirely of constant values. A constant expression gets evaluated at compile time, rather than at runtime, leading to more efficient code. Using constant expressions can optimize your code's performance and guarantee that some values remain unchanged throughout the execution of a program.

Defining constant expressions is done through the const or constexpr keywords, or through preprocessor macros. For example, const int maxUsers = 100; defines a constant expression with a fixed value.

In the context of the given exercise, using the preprocessor directive #define SIZE 12 creates a constant expression that is replaced throughout the code during preprocessing. This operation guarantees that values like SIZE do not change and are consistently used, which is especially useful for array sizes, loop limits, and other constant values that your program might use.