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 << getIsTrue< isX<char>::ikIsX  >() << endl;
   cout << getIsTrue< isX<int8>::ikIsX  >() << endl;
   cout << getIsTrue< isX<uint8>::ikIsX  >() << 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 <>            struct isX<int   >  { typedef FalseType ikIsX; };
template <>            struct isX<unit32>  { 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::signed is provided. If this is true, char behaves as a signed type; otherwise, it behaves as an unsigned type.

The above is the detailed content of Are `char`, `signed char`, and `unsigned char` Truly Distinct in C ?. For more information, please follow other related articles on the PHP Chinese website!