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Reverted specialization for trivial types

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It caused too many problems. I left only specialiation for scalar types.
I will need to devise clever type traits for reconizing trivial types with working constructor.
This commit is contained in:
Andrzej Krzemienski
2017-11-04 17:42:22 +01:00
parent cbf3cd05af
commit 06dea2cb9b
11 changed files with 293 additions and 123 deletions
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6
doc/1A_on_performance.qbk
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@@ -10,7 +10,7 @@ Technical details aside, the memory layout of `optional<T>` for a generic `T` is
std::aligned_storage_t<sizeof(t), alignof(T)> _storage;
};
Lifetime of the `T` inside `_storage` is manually controlled with placement-`new`s and pseudo-destructor calls. However, for trivial `T`s we use a different way of storage, by simply holding a `T`:
Lifetime of the `T` inside `_storage` is manually controlled with placement-`new`s and pseudo-destructor calls. However, for scalar `T`s we use a different way of storage, by simply holding a `T`:
template <typename T>
class optional
@@ -19,7 +19,7 @@ Lifetime of the `T` inside `_storage` is manually controlled with placement-`new
T _storage;
};
We call it a ['direct] storage. This makes `optional<T>` a trivially-copyable type for trivial `T`s. This only works for compilers that support defaulted functions and type traits. On compilers without defaulted functions we still use the direct storage, but `optional<T>` is no longer recognized as trivially-copyable. On compilers that do not fully support type traits, we still use the direct storage for scalar types, but we leave the programmer a way of customizing her type, so that it is reconized by `optional` as trivial, by specializing type trait `boost::opitonal_config::is_type_trivial`:
We call it a ['direct] storage. This makes `optional<T>` a trivially-copyable type for scalar `T`s. This only works for compilers that support defaulted functions. On compilers without defaulted functions we still use the direct storage, but `optional<T>` is no longer recognized as trivially-copyable. Apart from scalar types, we leave the programmer a way of customizing her type, so that it is reconized by `optional` as candidate for optimized storage, by specializing type trait `boost::opitonal_config::optional_uses_direct_storage_for`:
struct X // not trivial
{
@@ -28,7 +28,7 @@ We call it a ['direct] storage. This makes `optional<T>` a trivially-copyable ty
namespace boost { namespace optional_config {
template <> struct is_type_trivial<X> : boost::true_type {};
template <> struct optional_uses_direct_storage_for<X> : boost::true_type {};
}}