U>>Работы с сырыми указателями — это всегда опасно

U>>  const char* host = "domainname.com";
U>>  const char* ptr = host;
U>>  while (*ptr && *ptr != '.')
U>>  {                
U>>    ++ptr;
U>>  }
U>>  unsigned int hostLenght = ptr - host;
U>>

N>const char* host = "domainname.com";

N>const char *ptr = host;
N>const char *end = host + strlen(host);

N>while(ptr != end && *ptr != '.')
N>{                
N>    ptr++;
N>}

N>unsigned int hostLenght = (unsigned int)abs(ptr - host);
N>

unsigned int hostLenght = (unsigned int)abs(ptr - host);

const char *end = host + strlen(host);

  const char* host = "domainname.com";
  const char* ptr = host;
  while (*ptr && *ptr != '.')
  {                
    ++ptr;
  }
  unsigned int hostLenght = ptr - host;

ptrdiff_t hostLenght = ptr - host;

8.3.4 Arrays
1.
<skip>
An object of array type contains a contiguously allocated nonempty set of N sub-objects of type T

5.7 Additive operators
<skip>
6.
When two pointers to elements of the same array object are subtracted, the result is the difference of the
subscripts of the two array elements. The type of the result is an implementation-defined signed integral
type; this type shall be the same type that is defined as ptrdiff_t in the <cstddef> header (18.1). As
with any other arithmetic overflow, if the result does not fit in the space provided, the behavior is undefined.
In other words, if the expressions P and Q point to, respectively, the i-th and j-th elements of an
array object, the expression (P)-(Q) has the value i–j provided the value fits in an object of type
ptrdiff_t. Moreover, if the expression P points either to an element of an array object or one past the
last element of an array object, and the expression Q points to the last element of the same array object, the
expression ((Q)+1)-(P) has the same value as ((Q)-(P))+1 and as -((P)-((Q)+1)), and has
the value zero if the expression P points one past the last element of the array object, even though the
expression (Q)+1 does not point to an element of the array object. Unless both pointers point to elements
of the same array object, or one past the last element of the array object, the behavior is undefined.

    const char* domain = strchr( host, '.' );
    std::ptrdiff_t hostLenght = std::distance( host, domain );

U>Работы с сырыми указателями — это всегда опасно

U>  const char* host = "domainname.com";
U>  const char* ptr = host;
U>  while (*ptr && *ptr != '.')
U>  {                
U>    ++ptr;
U>  }
U>  unsigned int hostLenght = ptr - host;
U>

const char* host = "domainname.com";

const char *ptr = host;
const char *end = host + strlen(host);

while(ptr != end && *ptr != '.')
{                
    ptr++;
}

unsigned int hostLenght = (unsigned int)abs(ptr - host);

const char* host = "domainname.com";

const char *ptr = host;

while(*ptr != '.')
{                
    ptr++;
}

unsigned int hostLenght = (unsigned int)abs(ptr - host);

N>const char* host = "domainname.com";

N>const char *ptr = host;

N>while(*ptr != '.')
N>{                
N>    ptr++;
N>}

N>unsigned int hostLenght = (unsigned int)abs(ptr - host);
N>

std::string host("domainname.com");
size_t hostLenght = host.find('.');
//  не забудь проверить на npos

const char* host = "domainname.com";
const char* domain = strchr(host, '.');
unsigned int host_length = domain?(unsigned int)(domain - host):0;