stringpiece.h | Explore in Territory

// Protocol Buffers - Google's data interchange format
// Copyright 2008 Google Inc.  All rights reserved.
// https://developers.google.com/protocol-buffers/
//
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//
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//
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// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

// A StringPiece points to part or all of a string, Cord, double-quoted string
// literal, or other string-like object.  A StringPiece does *not* own the
// string to which it points.  A StringPiece is not null-terminated.
//
// You can use StringPiece as a function or method parameter.  A StringPiece
// parameter can receive a double-quoted string literal argument, a "const
// char*" argument, a string argument, or a StringPiece argument with no data
// copying.  Systematic use of StringPiece for arguments reduces data
// copies and strlen() calls.
//
// Prefer passing StringPieces by value:
//   void MyFunction(StringPiece arg);
// If circumstances require, you may also pass by const reference:
//   void MyFunction(const StringPiece& arg);  // not preferred
// Both of these have the same lifetime semantics.  Passing by value
// generates slightly smaller code.  For more discussion, see the thread
// go/stringpiecebyvalue on c-users.
//
// StringPiece is also suitable for local variables if you know that
// the lifetime of the underlying object is longer than the lifetime
// of your StringPiece variable.
//
// Beware of binding a StringPiece to a temporary:
//   StringPiece sp = obj.MethodReturningString();  // BAD: lifetime problem
//
// This code is okay:
//   string str = obj.MethodReturningString();  // str owns its contents
//   StringPiece sp(str);  // GOOD, because str outlives sp
//
// StringPiece is sometimes a poor choice for a return value and usually a poor
// choice for a data member.  If you do use a StringPiece this way, it is your
// responsibility to ensure that the object pointed to by the StringPiece
// outlives the StringPiece.
//
// A StringPiece may represent just part of a string; thus the name "Piece".
// For example, when splitting a string, vector<StringPiece> is a natural data
// type for the output.  For another example, a Cord is a non-contiguous,
// potentially very long string-like object.  The Cord class has an interface
// that iteratively provides StringPiece objects that point to the
// successive pieces of a Cord object.
//
// A StringPiece is not null-terminated.  If you write code that scans a
// StringPiece, you must check its length before reading any characters.
// Common idioms that work on null-terminated strings do not work on
// StringPiece objects.
//
// There are several ways to create a null StringPiece:
//   StringPiece()
//   StringPiece(nullptr)
//   StringPiece(nullptr, 0)
// For all of the above, sp.data() == nullptr, sp.length() == 0,
// and sp.empty() == true.  Also, if you create a StringPiece with
// a non-null pointer then sp.data() != nullptr.  Once created,
// sp.data() will stay either nullptr or not-nullptr, except if you call
// sp.clear() or sp.set().
//
// Thus, you can use StringPiece(nullptr) to signal an out-of-band value
// that is different from other StringPiece values.  This is similar
// to the way that const char* p1 = nullptr; is different from
// const char* p2 = "";.
//
// There are many ways to create an empty StringPiece:
//   StringPiece()
//   StringPiece(nullptr)
//   StringPiece(nullptr, 0)
//   StringPiece("")
//   StringPiece("", 0)
//   StringPiece("abcdef", 0)
//   StringPiece("abcdef"+6, 0)
// For all of the above, sp.length() will be 0 and sp.empty() will be true.
// For some empty StringPiece values, sp.data() will be nullptr.
// For some empty StringPiece values, sp.data() will not be nullptr.
//
// Be careful not to confuse: null StringPiece and empty StringPiece.
// The set of empty StringPieces properly includes the set of null StringPieces.
// That is, every null StringPiece is an empty StringPiece,
// but some non-null StringPieces are empty Stringpieces too.
//
// All empty StringPiece values compare equal to each other.
// Even a null StringPieces compares equal to a non-null empty StringPiece:
//  StringPiece() == StringPiece("", 0)
//  StringPiece(nullptr) == StringPiece("abc", 0)
//  StringPiece(nullptr, 0) == StringPiece("abcdef"+6, 0)
//
// Look carefully at this example:
//   StringPiece("") == nullptr
// True or false?  TRUE, because StringPiece::operator== converts
// the right-hand side from nullptr to StringPiece(nullptr),
// and then compares two zero-length spans of characters.
// However, we are working to make this example produce a compile error.
//
// Suppose you want to write:
//   bool TestWhat?(StringPiece sp) { return sp == nullptr; }  // BAD
// Do not do that.  Write one of these instead:
//   bool TestNull(StringPiece sp) { return sp.data() == nullptr; }
//   bool TestEmpty(StringPiece sp) { return sp.empty(); }
// The intent of TestWhat? is unclear.  Did you mean TestNull or TestEmpty?
// Right now, TestWhat? behaves likes TestEmpty.
// We are working to make TestWhat? produce a compile error.
// TestNull is good to test for an out-of-band signal.
// TestEmpty is good to test for an empty StringPiece.
//
// Caveats (again):
// (1) The lifetime of the pointed-to string (or piece of a string)
//     must be longer than the lifetime of the StringPiece.
// (2) There may or may not be a '\0' character after the end of
//     StringPiece data.
// (3) A null StringPiece is empty.
//     An empty StringPiece may or may not be a null StringPiece.

#ifndef GOOGLE_PROTOBUF_STUBS_STRINGPIECE_H_
#define GOOGLE_PROTOBUF_STUBS_STRINGPIECE_H_

#include <assert.h>
#include <stddef.h>
#include <string.h>
#include <iosfwd>
#include <limits>
#include <string>

#if defined(__cpp_lib_string_view)
#include <string_view>
#endif

#include <google/protobuf/stubs/hash.h>

#include <google/protobuf/port_def.inc>

namespace google {
namespace protobuf {
namespace stringpiece_internal {

class PROTOBUF_EXPORT StringPiece { … };

// This large function is defined inline so that in a fairly common case where
// one of the arguments is a literal, the compiler can elide a lot of the
// following comparisons.
inline bool operator==(StringPiece x, StringPiece y) { … }

inline bool operator!=(StringPiece x, StringPiece y) { … }

inline bool operator<(StringPiece x, StringPiece y) { … }

inline bool operator>(StringPiece x, StringPiece y) { … }

inline bool operator<=(StringPiece x, StringPiece y) { … }

inline bool operator>=(StringPiece x, StringPiece y) { … }

// allow StringPiece to be logged
extern std::ostream& operator<<(std::ostream& o, StringPiece piece);

}  // namespace stringpiece_internal

StringPiece;

}  // namespace protobuf
}  // namespace google

GOOGLE_PROTOBUF_HASH_NAMESPACE_DECLARATION_START
template<> struct hash<StringPiece> { … };
GOOGLE_PROTOBUF_HASH_NAMESPACE_DECLARATION_END

#include <google/protobuf/port_undef.inc>

#endif  // STRINGS_STRINGPIECE_H_
chromium/third_party/protobuf/src/google/protobuf/stubs/stringpiece.h
