Why Use Meaningless do-while and if-else statements in C++ Macros?

Have you ever wondered why C++ macros sometimes include seemingly meaningless do-while(0) and if-else statements? You may dismiss them as worthless, but if you aren’t careful, they could cause undesirable bugs and undefined behavior in your C++ code. In this article, we’ll explore the uses of these constructs and how they help keep your code safe, eliminate multiple evaluations, and strengthen macro programming in C++. 

Table of Contents:

• What are Macros in C++?
  1. Why Use do-while(0) in Macros?
  2. Why Use if-else in Macros?
• Combining do-while(0) and if-else for Robust Macros
• Conclusion

What are Macros in C++? 

In C++, the macros are defined using preprocessor directives like #define. Without any type of safety or inherent scope, the macros are text replacements. If macros are used in expressions, these may sometimes lead to undefined behavior and cause serious issues. The macros are combined with do-while or if-else to ensure the predictable execution of a program. 

A simple example for Macros : 

Output: 

The above code defines a macro MAX(a, b) using a ternary operator the macro returns the larger of two values. In the main function, the comparison between the x and y values takes place. Lastly, it prints the maximum value, which is 20. 

1. What is The Use of do-while(0) in Macros?

In C++, the do-while(0) is used in macros to make sure the macro behaves like a single statement, even in conditional statements.

• Ensures Single Statement Semantics: Without using extra braces the single statements allow the safety of macros to be used inside if-statements.
• Avoids Unexpected Behavior: When we are using conditional statements, it prevents partial execution. 
• Encapsulates Multiple Statements: Makes sure that all macro statements are executed as blocks.

Example: 

Output: 

In the above code, using do-while(0), the PRINT_NUM(x) safely prints the number by making sure it is a single statement. In the main() function, the if condition checks the number; if it is true, it prints  “Number: 10”, and the else block never executes. 

2. What is the Use of if-else in Macros?

In C++, using an if-else statement inside macros ensures the conditional execution, and also the dangling else problem in macros occurs when an if-else statement is not used properly due to missing braces, which is why do { … } while(0) is used for proper scoping.

This maintains the correct behavior when it is expanded. 

• In functions like macros, If-else prevents the multiple evaluation of expressions.
• If-else also handles the inline return values of macros.
• If-else ensures the correct expansion in conditional execution.

Example: 

Output: 

In the above code, to prevent multiple executions, the macro MAX(a, b) safely stores a and b in temporary variables and finds the maximum. While returning the larger value, it increments the x++ and y++.

Combining do-while(0) and if-else for Robust Macros

In macros, combining both do-while(0) and if-else makes sure that they behave as a single statement, and without any further syntax issues, it executes conditionally. The do-while(0) ensures the safety of multi-line macros that are structured inside the if statements. Now, when the macro is disabled, the if-else prevents the undefined side effects.

Example: 

Output: 

In the above example, the macro LOG(msg) prints the debug messages when the DEBUG is 1. Inside the if block, the do-while makes sure it acts as a single statement. If DEBUG is 0, it prints the else (void)0; and prevents the syntax issues. LOG(msg) is expanded in a conditional statement.

Conclusion

In macros, using do-while(0) and if-else statements makes sure they behave as single statements and also prevents the undefined behavior in conditional expressions. The do-while(0) ensures the safety of multi-line macros that are structured inside the if statements, whereas if-else statements ensure correct execution by avoiding multiple evaluations. But combining both techniques makes the macros efficient and prevents syntax errors. 

You can learn more about C++ in the C++ programming language and also explore C++ Interview Questions prepared by industry experts.

Why Use Meaningless do-while and if-else statements in C++ Macros?- FAQs