Is `isdigit(c)` Designed for `char` or `int` Input? A Detailed Examination.

isdigit(c) - What Data Type Should the Input Variable Be?

Introduction:
When working with character-based input, it's essential to determine if a given input is a digit. The isdigit() function is a commonly used tool for this purpose, but it raises the question: should the input variable be a character (char) or an integer (int)?

Character Input and Stream State:
Cin is used to read character input. However, it's crucial to check if the stream is valid before using the input. If the stream is closed, cin may return without modifying the input variable, leaving it uninitialized.

isdigit() Contract and Implementation:
The C function isdigit() expects an integer argument. This design stems from the original C function getchar(), which returns an int to indicate character codes and error status. As a result, isdigit() assumes the input is an unsigned char cast to int.

Signedness of Char:
In C, plain char can be either signed or unsigned, depending on the compiler's implementation. In most cases, it's signed. This introduces a compatibility issue when using getchar() and the isdigit() family of functions.

Negative values in signed char may conflict with the EOF indicator value, causing potential overlap and undefined behavior. To avoid this, isdigit() should be called with the input cast as (unsigned char).

Code Example:

<code class="c++">#include <iostream>
#include <ctype.h>

int main()
{
    char c;

    cout > c;

    if (isdigit((unsigned char)c))
        cout </ctype.h></iostream></code>

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