Why Does Casting to `void` in C Suppress Warnings While Other Casts Don't?

The Intriguing Effects of Casting to void

In C , the act of suppressing warnings about unused variables using (void)x is widespread. However, casting to void exhibits peculiar behavior when compared to casting to other types.

Compiler Warnings and Casts

Consider the following code snippet:

int main()
{
    int x;
    (short)x;
    (void)x;
    (int)x;
}

Compiling this code with warnings enabled using -Wall -Wextra yields the following:

warning: statement has no effect [-Wunused-value]
     (short)x;
warning: statement has no effect [-Wunused-value]
     (int)x;

This suggests that casting to void behaves differently from casting to other types.

Possible Explanations

Two possible explanations for this difference arise:

1. (void)x is simply a convention to suppress warnings, while other casts have no such effect, despite all statements being equally ineffective.
2. This behavior is linked to the fact that void x; is an invalid declaration, unlike short x;.

The Correct Answer

The correct explanation is the first one. Casting to void is a dedicated warning suppression mechanism. The Standard explicitly states in §5.2.9/4:

Any expression can be explicitly converted to type “cv void.” The
expression value is discarded.

Therefore, the casting to void has no practical effect on the program's execution. It merely serves to silence warnings about potentially unused variables.

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