P2321R2 adds new conditionally explicit constructors to std::tuple which
we'll concisely implement in a subsequent patch using explicit(bool), like
in our C++20 std::pair implementation. But before we can do that, this
patch first adds members to _TupleConstraints that test for constructibility
and convertibility separately; we'll use the first in the new constructors'
constraints, and the second in their explicit specifier.
In passing, this patch also redefines the existing predicates
__is_ex/implicitly_constructible in terms of these new members. This
seems to reduce compile time and memory usage by about 10% for large
tuples when using the relevant constructors constrained by
_Explicit/_ImplicitCtor (since we no longer have to redundantly expand
and process is_constructible<_Types, _UTypes>... twice for each pair of
such constructors). In order to retain maximal short circuiting, do
this only when constexpr if is available.
Tested on x86_64-pc-linux-gnu, does this look OK for trunk?
libstdc++-v3/ChangeLog:
* include/std/tuple (_TupleConstraints::__convertible): Define
for C++17.
(_TupleConstraints::__constructible): Likewise.
(_TupleConstraints::__is_explicitly_constructible): For C++17
define this in terms of __convertible and __constructible
using constexpr if.
(_TupleConstraints::__is_implicitly_constructible): Likewise.
---
libstdc++-v3/include/std/tuple | 22 ++++++++++++++++++++++
1 file changed, 22 insertions(+)
diff --git a/libstdc++-v3/include/std/tuple b/libstdc++-v3/include/std/tuple
index 6d0060a191c..d0c168fd7e2 100644
--- a/libstdc++-v3/include/std/tuple
+++ b/libstdc++-v3/include/std/tuple
@@ -553,15 +553,31 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
template<bool, typename... _Types>
struct _TupleConstraints
{
+#if __cplusplus >= 201703L
+ template<typename... _UTypes>
+ static constexpr bool __constructible
+ = __and_v<is_constructible<_Types, _UTypes>...>;
+
+ template<typename... _UTypes>
+ static constexpr bool __convertible
+ = __and_v<is_convertible<_UTypes, _Types>...>;
+#endif
+
// Constraint for a non-explicit constructor.
// True iff each Ti in _Types... can be constructed from Ui in _UTypes...
// and every Ui is implicitly convertible to Ti.
template<typename... _UTypes>
static constexpr bool __is_implicitly_constructible()
{
+#if __cplusplus >= 201703L
+ if constexpr (__constructible<_UTypes...>)
+ return __convertible<_UTypes...>;
+ return false;
+#else
return __and_<is_constructible<_Types, _UTypes>...,
is_convertible<_UTypes, _Types>...
>::value;
+#endif
}
// Constraint for a non-explicit constructor.
@@ -570,9 +586,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
template<typename... _UTypes>
static constexpr bool __is_explicitly_constructible()
{
+#if __cplusplus >= 201703L
+ if constexpr (__constructible<_UTypes...>)
+ return !__convertible<_UTypes...>;
+ return false;
+#else
return __and_<is_constructible<_Types, _UTypes>...,
__not_<__and_<is_convertible<_UTypes, _Types>...>>
>::value;
+#endif
}
static constexpr bool __is_implicitly_default_constructible()
--
2.37.2.382.g795ea8776b
