Are `char`, `signed char`, and `unsigned char` Truly Distinct in C ?

Character Types in C : Distinct or Equivalent?

In C , the behavior of character types (char) can sometimes differ from that of signed and unsigned integers, leading to confusion. Specifically, the following code demonstrates this difference:

<code class="cpp">#include <iostream>

typedef   signed char       int8;
typedef unsigned char      uint8;

struct TrueType {};
struct FalseType {};

template <typename t>
struct isX
{
   typedef typename T::ikIsX ikIsX;
};

template             struct isX<char>  { typedef FalseType ikIsX; };
template             struct isX<int8>  { typedef FalseType ikIsX; };
template             struct isX<uint8>  { typedef FalseType ikIsX; };

template <typename t> bool getIsTrue();
template            bool getIsTrue<truetype>() { return true; }
template            bool getIsTrue<falsetype>() { return false; }

int main(int, char **t )
{
   cout ::ikIsX  >() ::ikIsX  >() ::ikIsX  >() <p>This code compiles but produces different results for char than for int8 and uint8. This is because C   treats char, signed char, and unsigned char as three distinct types.</p>
<p>In contrast, int and uint32 are equivalent types:</p>
<pre class="brush:php;toolbar:false"><code class="cpp">template             struct isX<int>  { typedef FalseType ikIsX; };
template             struct isX<unit32>  { typedef FalseType ikIsX; };</unit32></int></code>

This distinction stems from the fact that char has historically been used for both representing characters and storing numeric values. As a result, C maintains backward compatibility by treating plain char as a separate type, distinct from int.

To determine which of the two representations char uses, the implementation-defined typedef char_traits::signed is provided. If this is true, char behaves as a signed type; otherwise, it behaves as an unsigned type.

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