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Bug 852393 - Specialize nsAutoTArray<T, 0> to be an nsTArray - r=froydnj

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This commit is contained in:
Benoit Jacob 2014-05-29 11:59:37 -04:00
parent 02d7f6f1f9
commit 0746eb9600
1 changed files with 19 additions and 0 deletions
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19
xpcom/glue/nsTArray.h
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@ -1696,6 +1696,7 @@ public:
template <class TArrayBase, size_t N>
class nsAutoArrayBase : public TArrayBase
{
static_assert(N != 0, "nsAutoArrayBase<TArrayBase, 0> should be specialized");
public:
typedef nsAutoArrayBase<TArrayBase, N> self_type;
typedef TArrayBase base_type;
@ -1756,6 +1757,24 @@ private:
};
};
//
// Specialization of nsAutoArrayBase<TArrayBase, N> for the case where N == 0.
// nsAutoArrayBase<TArrayBase, 0> behaves exactly like TArrayBase, but without
// this specialization, it stores a useless inline header.
//
// We do have many nsAutoArrayBase<TArrayBase, 0> objects in memory: about
// 2,000 per tab as of May 2014. These are typically not explicitly
// nsAutoArrayBase<TArrayBase, 0> but rather nsAutoArrayBase<TArrayBase, N>
// for some value N depending on template parameters, in generic code.
//
// For that reason, we optimize this case with the below partial specialization,
// which ensures that nsAutoArrayBase<TArrayBase, 0> is just like TArrayBase,
// without any inline header overhead.
//
template <class TArrayBase>
class nsAutoArrayBase<TArrayBase, 0> : public TArrayBase
{};
//
// nsAutoTArray<E, N> is an infallible vector class with N elements of inline
// storage. If you try to store more than N elements inside an