How To Ignore Return Value In C++

In C++, functions often return values that can be used to determine the success of an operation, provide computed data, or indicate error states. However, there are scenarios where you might intentionally choose to ignore these return values. Whether you're dealing with legacy code, third-party libraries, or simply want to streamline your code, understanding how to properly ignore return values in C++ is essential. This guide explores various techniques and best practices to do so effectively and safely.

Understanding When to Ignore Return Values

Before learning how to ignore return values, it's important to understand when it is appropriate. Ignoring return values can be acceptable in situations such as:

• When the return value is not critical: For example, a function that logs information but doesn't affect program flow.
• For performance considerations: When checking return values incurs unnecessary overhead in performance-critical sections.
• During cleanup or fire-and-forget operations: When the outcome doesn't impact subsequent code.

However, ignoring return values without proper consideration can lead to bugs, unhandled errors, or undefined behavior. Always evaluate whether the return value needs to be checked or can be safely ignored.

Explicitly Ignoring Return Values in C++

The simplest way to ignore a return value in C++ is to assign it to (void). This approach explicitly indicates that the return value is intentionally disregarded, improving code readability and maintainability.

Using (void) Cast to Ignore Return Values

To explicitly ignore a function's return value, cast it to void. This syntax informs both the compiler and readers of the code that you are intentionally disregarding the return value.

<!-- Example -->
int result = someFunction();
(void)result; // Explicitly ignore result

// Or directly:
(void)someFunction();

This method is portable and widely used, especially for functions returning non-void types.

Discarding Return Values Without Casting

In many cases, simply calling a function and not assigning its return value effectively ignores it. However, some compilers issue warnings if the return value is ignored, especially when the function's return value indicates important status information.

someFunction(); // Return value is ignored, may cause compiler warning

To suppress such warnings, prefer using the (void) cast method.

Ignoring Return Values in Specific Contexts

Different scenarios may require different approaches to ignore return values. Here are some common contexts:

1. Ignoring Return Values in Function Calls

If you want to call a function but do not need its return value, simply call it without assigning the result:

updateDatabase(); // Function called, return value ignored

To prevent compiler warnings, especially in strict compilation modes, consider casting:

(void)updateDatabase();

2. Ignoring Return Values in Lambda Expressions

When using lambda functions, sometimes the return value is irrelevant. You can simply omit the return statement or ignore it explicitly:

auto lambda = []() {
    someFunction(); // Ignoring return value
};

3. Ignoring Return Values in Macros and Inline Functions

Macros or inline functions can also return values that you may want to ignore. Use the same casting approach to make intentions clear:

#define CALL_AND_IGNORE(expr) (void)(expr)

// Usage:
CALL_AND_IGNORE(someFunction());

Best Practices for Ignoring Return Values

While ignoring return values can be necessary, it’s crucial to do so thoughtfully. Here are some best practices:

• Document your intent: Use comments or explicit casting to indicate that ignoring the return value is intentional.
• Be aware of potential side effects: Ensure that ignoring the return value doesn't hide errors or critical information.
• Use static analysis tools: Employ tools that warn about ignored return values, especially for functions where ignoring return values is risky.
• Follow coding standards: Adhere to project or organization coding guidelines regarding ignored return values.

Handling Return Values Safely

Sometimes, ignoring return values might be unsafe. To handle such cases gracefully, consider the following:

• Check return values explicitly: Use conditional statements to verify success or handle errors.
• Use exception handling: If the function throws exceptions on failure, wrapping calls in try-catch blocks ensures robustness.
• Log ignored return values: For operations where ignoring return values might lead to silent failures, implement logging or error reporting mechanisms.

Examples of Ignoring Return Values in Common C++ Functions

Here are some typical examples where ignoring return values is common:

// Ignoring return value of std::fstream::close()
std::ofstream file("example.txt");
file << "Hello, World!";
file.close(); // Ignoring the return value, which is usually safe

// Ignoring return value of system()
int result = system("ls -l");
(void)result; // Explicitly ignoring result

// Ignoring return value of a third-party library function
auto status = thirdPartyFunction();
if (status != SUCCESS) {
    // handle error
} else {
    // ignoring in some cases
    (void)status; // if really safe to ignore
}

Conclusion

Ignoring return values in C++ is straightforward but should be approached with caution. Using techniques like casting to void ensures clarity and signals intentional disregard, helping prevent accidental bugs and warnings. Always consider whether ignoring a return value is appropriate for your specific context, and adhere to best practices to maintain code safety and readability. Proper handling of return values ultimately leads to more reliable and maintainable C++ programs.

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