Unused variables in C/C: Why and how?

In C/C code review, there are often cases where variables are not used. This article will explore common reasons for unused variables and explain how to get the compiler to issue warnings and how to suppress specific warnings.

Causes of unused variables

There are many reasons why unused variables appear in the code:

1. Code flaws or errors: The most direct reason is that there are problems with the code itself, and the variables may not be needed at all, or they are needed but not used correctly.
2. Code refactoring: During the software development process, the code will be continuously modified and refactored, and some once important variables may be left behind and unused.
3. Reserved variables: Developers may predeclare some variables for future use, but they will not be used in the end.
4. Conditional compilation: Some variables may be used only under certain conditions (such as debug mode) and are ignored in other cases. For example:

 <code class="language-c  ">const auto value = compute_some_value(); const auto value_for_comparison_only = compute_same_value_differently(); assert(value == value_for_comparison_only);</code>

If compiled with -DNDEBUG , value_for_comparison_only may be marked as unused.

Detect unused variables

Different compilers and warning levels affect detection of unused variables. GCC and Clang use the -Wunused-variable option to enable unused variable warnings. The -Wall option usually contains this warning and can be disabled using -Wno-unused-variable . It is recommended to always use -Wall to selectively turn off specific warnings if necessary.

Suppress unused variable warnings

While it is recommended to enable as many warnings as possible, in some cases it may be necessary to selectively turn off warnings for specific unused variables. Common methods include:

1. Cast: cast variable to void :

 <code class="language-c  ">object unused_object; (void)unused_object;</code>

This eliminates the warning, but is not clear enough.

2. Use macro: Define a macro to explicitly indicate that the variable is not used:

 <code class="language-c  ">#define unused(x) (void)(x); // ... object unused_object; unused(unused_object);</code>

This improves the readability and maintainability of the code.

3. Use attributes: Clang and GCC support __attribute__((unused)) attribute, and C 17 supports the [[maybe_unused]] attribute:

 <code class="language-c  ">object unused_object2 __attribute__((unused)) = x; // 声明后使用[[maybe_unused]] object unused_object1 = x; // 声明前使用</code>

These properties explicitly inform the compiler (and developers) that the variables may not be used. __attribute__((unused)) even issues a warning when a variable is used unexpectedly. Individuals prefer to use [[maybe_unused]] , especially in conditional compilation.

Keep unused variables

During the development and debug phases, it is sometimes beneficial to retain unused variables. For example, it might represent legacy parts of past code, or be used for debugging purposes:

 <code class="language-c  ">auto unused_variable __attribute__((unused)) = complicated_calculation(arg1, arg2, arg3);</code>

Even if the result is not used, it can be retained as a potential debugging point.

All in all, a reasonable handling of unused variables is essential to keeping your code tidy and efficient. Choosing the right strategy depends on the situation.

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