nsGkAtoms.h - mozsearch

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/* This Source Code Form is subject to the terms of the Mozilla Public

 * License, v. 2.0. If a copy of the MPL was not distributed with this

 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

#ifndef nsGkAtoms_h_

#define nsGkAtoms_h_

#include "mozilla/StaticAtoms.h"

#include "nsAtom.h"

// Static atoms are structured carefully to satisfy a lot of constraints.

//

// - We have ~2700 static atoms.

//

// - We want them to be constexpr and not use relocations, so they end up in

//   .rodata, and thus shared between processes, minimizing memory usage.

//

// - We need them to be in an array, so we can iterate over them (for

//   registration / lookups / representation as an index).

//

// - Each static atom has a string literal associated with it. We can't use a

//   pointer to the string literal because then the atoms won't end up in

//   .rodata. Therefore the string literals and the atoms must be arranged in a

//   way such that a numeric index can be used instead. This numeric index

//   (nsStaticAtom::mStringOffset) must be computable at compile-time to keep

//   the static atom constexpr. It should also not be too large (a uint32_t is

//   reasonable).

//

// - As well as accessing each static atom via array indexing, we need an

//   individual pointer, e.g. nsGkAtoms::foo. We want this to be constexpr so

//   it doesn't take up any space in memory.

//

// - We want to minimize the header size / complexity, but the individual

//   pointers for the static atoms must be in a .h file so they are public.

//

// StaticAtoms.h below defines static atoms in a way that satisfies these

// constraints: A GkAtoms struct which arranges the strings and atoms together,

// defined in nsGkAtoms.cpp.

namespace nsGkAtoms {

inline nsStaticAtom* GetAtomByIndex(size_t aIndex) {

  MOZ_ASSERT(aIndex < kStaticAtomCount);

  return const_cast<nsStaticAtom*>(&detail::gGkAtoms.mAtoms[aIndex]);

inline size_t IndexOf(const nsStaticAtom* atom) {

  nsStaticAtom* firstAtom = GetAtomByIndex(0);

  size_t ret = atom - firstAtom;

  MOZ_ASSERT(ret < kStaticAtomCount);

  return ret;

}  // namespace nsGkAtoms

inline bool nsAtom::IsEmpty() const { return this == nsGkAtoms::_empty; }

#endif /* nsGkAtoms_h_ */