August 01, 2003
Generalized String Manipulation: Access Shims and Type Tunneling
NULL Strings
NULL Strings
The standard library string
c_str() method returns "a null-terminated array
of characters representing the string's contents" [2]. It is important to note
that it does not qualify this definition with whether the represented string is
empty, so the implementation of the string may store a null pointer when an instance
is in an "empty" state. But when called in that state it must still return a valid
string; obviously it should return the empty string. A typical implementation
would be like the following:
template <...>
inline char_type const *string_class<...>::c_str() const
{
return m_buffer != 0 ? m_buffer : "";
}
With C-strings, ie.,
char const *, an empty string can come in two forms,
the empty string (which is a pointer to an array of one character, with value
0), or it can be the null string (which is the null pointer). This is a serious
disparity. On many architectures, de-referencing the null pointer causes an access
violation, and it is in all cases undefined, so it is something we want to avoid.
The strategy I chose for the string access shims was to implement c_str_ptr
to match the semantics of c_str(), hence it never returns a null pointer.
What this means is that any algorithm that uses the c_str_ptr shims can
rely on the fact that it will never be presented with a null string.
However, some uses of shims might require a NULL pointer to be returned when
the string is empty. This is supported by the shim c_str_ptr_null. Each
c_str_ptr() shim in the STLSoft libraries has a matching c_str_ptr_null()
shim, some of which are shown in Listing 11.
Previous Page |
1
|
2
|
3
|
4
|
5
|
6
|
7
|
8
|
9
|
10
|
11
|
12
|
13
|
14
|
15
|
16
|
17
|
18
|
19
|
20
Next Page
