Both for quick comparison as for quick building of lists of atoms, it
is desirable to provide access to Prolog's atom-table, mapping handles
to unique string-constants. If the handles of two atoms are different it
is guaranteed they represent different text strings.
Suppose we want to test whether a term represents a certain atom,
this interface presents a large number of alternatives:
Example:
PREDICATE(test, 1)
{ if ( A1 == "read" )
...;
}
This writes easily and is the preferred method is performance is not
critical and only a few comparisons have to be made. It validates
A1 to be a term-reference representing text (atom, string,
integer or float) extracts the represented text and uses strcmp()
to match the strings.
Example:
static PlAtom ATOM_read("read");
PREDICATE(test, 1)
{ if ( A1 == ATOM_read )
...;
}
This case raises a type_error if A1 is not an
atom. Otherwise it extacts the atom-handle and compares it to the
atom-handle of the global PlAtom object. This approach is
faster and provides more strict type-checking.
Example:
static PlAtom ATOM_read("read");
PREDICATE(test, 1)
{ PlAtom a1(A1);
if ( a1 == ATOM_read )
...;
}
This approach is basically the same as section
2.11.12.2, but in nested if-then-else the extraction of the atom
from the term is done only once.
Example:
PREDICATE(test, 1)
{ PlAtom a1(A1);
if ( a1 == "read" )
...;
}
This approach extracts the atom once and for each test extracts the
represented string from the atom and compares it. It avoids the need for
global atom constructors.
- PlAtom :: PlAtom(atom_t handle)
- Create from C-interface atom handle (
atom_t). Used
internally and for integration with the C-interface.
- PlAtom :: PlAtom(const char_t *text)
- PlAtom :: PlAtom(const wchar *text)
- PlAtom :: PlAtom(const std::string& text)
- PlAtom :: PlAtom(const std::wstring& text)
- Create an atom from a string. The text is copied if a new
atom is created. See PL_new_atom(), PL_new_atom_wchars(),
PL_new_atom_nchars(), PL_new_atom_wchars().
- PlAtom :: PlAtom(const PlTerm &)
- If t represents an atom, the new instance represents this
atom. Otherwise a
type_error is thrown.
- int PlAtom::operator
==(const wchar_t *text)
- int PlAtom::operator
==(const char *text)
- int PlAtom::operator
==(const std::string& text)
- int PlAtom::operator
==(const std::wstring& text)
- Yields
true if the atom represents text, false
otherwise. Performs a strcmp() or similar for this.
- int PlAtom::operator
==(const PlAtom &)
- Compares the two atom-handles, returning
true or
false. Because atoms are unique, there is no need to use strcmp()
for this.
- int PlAtom::operator
!=(const wchar_t *text)
- int PlAtom::operator
!=(const char *text)
- int PlAtom::operator
!=(const std::string& text)
- int PlAtom::operator
!=(const std::wstring& text)
- int PlAtom::operator
!=(const PlAtom &)
- The inverse of the
==
- bool is_valid()
- Verifies that the handle is valid. This can be used after calling a
function that returns an atom handle, to check that a new atom was
created.
- void reset()
- Sets the handle to an invalid valid - a subsequent call to is_null()
will return
true.
- const std::string as_string(PlEncoding
enc=EncLocale)
- Returns the string representation of the atom.29If
you wish to return a
char* from a function, you should not
do return t.as_string().c_str()
because that will return a pointer into the stack (Gnu C++ or Clang
options -Wreturn-stack-address or -Wreturn-local-addr)
can sometimes catch this, as can the runtime address sanitizer
when run with detect_stack_use_after_return=1.
This does not quote or escape any characters that would need to be
escaped if the atom were to be input to the Prolog parser. The possible
values for enc are:
EncLatin1 - throws an exception if cannot be
represented in ASCII.EncUTF8EncLocale - uses the locale to determine the
representation.
- const std:wstring as_wstring()
- Returns the string representation of the atom. This does not quote or
escape any characters that would need to be escaped if the atom were to
be input to the Prolog parser.
- void register_atom()
- See PL_register_atom().
- void unregister_atom()
- See PL_unregister_atom().
- void* blob_data(size_t
*len, struct PL_blob_t **type)
- See PL_blob_data().
