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 &lt;iostream&gt;

typedef   signed char       int8;
typedef unsigned char      uint8;

struct TrueType {};
struct FalseType {};

template &lt;typename T&gt;
struct isX
{
   typedef typename T::ikIsX ikIsX;
};

template &lt;&gt;            struct isX&lt;char  &gt;  { typedef FalseType ikIsX; };
template &lt;&gt;            struct isX&lt;int8  &gt;  { typedef FalseType ikIsX; };
template &lt;&gt;            struct isX&lt;uint8 &gt;  { typedef FalseType ikIsX; };

template &lt;typename T&gt; bool getIsTrue();
template &lt;&gt;           bool getIsTrue&lt;TrueType&gt;() { return true; }
template &lt;&gt;           bool getIsTrue&lt;FalseType&gt;() { return false; }

int main(int, char **t )
{
   cout &lt;&lt; getIsTrue&lt; isX&lt;char&gt;::ikIsX  &gt;() &lt;&lt; endl;
   cout &lt;&lt; getIsTrue&lt; isX&lt;int8&gt;::ikIsX  &gt;() &lt;&lt; endl;
   cout &lt;&lt; getIsTrue&lt; isX&lt;uint8&gt;::ikIsX  &gt;() &lt;&lt; endl;
}</code>

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.

In contrast, int and uint32 are equivalent types:

<code class="cpp">template &lt;&gt;            struct isX&lt;int   &gt;  { typedef FalseType ikIsX; };
template &lt;&gt;            struct isX&lt;unit32&gt;  { typedef FalseType ikIsX; };</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<char>::signed is provided. If this is true, char behaves as a signed type; otherwise, it behaves as an unsigned type.

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