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Implement LWG 2488 - Make the placeholders constexpr. 

This patch makes the bind placeholders in std::placeholders both (1) const and
(2) constexpr (See below).

This is technically a breaking change for any code using the placeholders
outside of std::bind and depending on them being non-const. However I don't
think this will break any real world code.

(1) Previously the placeholders were non-const extern globals in all
dialects. This patch changes these extern globals to be const in all dialects.
Since the cv-qualifiers don't participate in name mangling for globals this
is an ABI compatible change.

(2) Make the placeholders constexpr in C++11 and beyond. Although LWG 2488 only
applies to C++17 I don't see any reason not to backport this change.

llvm-svn: 273824
This commit is contained in:
Eric Fiselier 2016-06-26 21:01:34 +00:00
parent a9fb19d3a5
commit 555330719a
4 changed files with 63 additions and 21 deletions
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33
libcxx/include/functional
View File

@ -1980,16 +1980,29 @@ namespace placeholders
template <int _Np> struct __ph {};
_LIBCPP_FUNC_VIS extern __ph<1> _1;
_LIBCPP_FUNC_VIS extern __ph<2> _2;
_LIBCPP_FUNC_VIS extern __ph<3> _3;
_LIBCPP_FUNC_VIS extern __ph<4> _4;
_LIBCPP_FUNC_VIS extern __ph<5> _5;
_LIBCPP_FUNC_VIS extern __ph<6> _6;
_LIBCPP_FUNC_VIS extern __ph<7> _7;
_LIBCPP_FUNC_VIS extern __ph<8> _8;
_LIBCPP_FUNC_VIS extern __ph<9> _9;
_LIBCPP_FUNC_VIS extern __ph<10> _10;
#if defined(_LIBCPP_CXX03_LANG) || defined(_LIBCPP_BUILDING_BIND)
_LIBCPP_FUNC_VIS extern const __ph<1> _1;
_LIBCPP_FUNC_VIS extern const __ph<2> _2;
_LIBCPP_FUNC_VIS extern const __ph<3> _3;
_LIBCPP_FUNC_VIS extern const __ph<4> _4;
_LIBCPP_FUNC_VIS extern const __ph<5> _5;
_LIBCPP_FUNC_VIS extern const __ph<6> _6;
_LIBCPP_FUNC_VIS extern const __ph<7> _7;
_LIBCPP_FUNC_VIS extern const __ph<8> _8;
_LIBCPP_FUNC_VIS extern const __ph<9> _9;
_LIBCPP_FUNC_VIS extern const __ph<10> _10;
#else
constexpr __ph<1> _1{};
constexpr __ph<2> _2{};
constexpr __ph<3> _3{};
constexpr __ph<4> _4{};
constexpr __ph<5> _5{};
constexpr __ph<6> _6{};
constexpr __ph<7> _7{};
constexpr __ph<8> _8{};
constexpr __ph<9> _9{};
constexpr __ph<10> _10{};
#endif // defined(_LIBCPP_CXX03_LANG) || defined(_LIBCPP_BUILDING_BIND)
} // placeholders

21
libcxx/src/bind.cpp
View File

@ -7,6 +7,7 @@
//
//===----------------------------------------------------------------------===//
#define _LIBCPP_BUILDING_BIND
#include "functional"
_LIBCPP_BEGIN_NAMESPACE_STD
@ -14,16 +15,16 @@ _LIBCPP_BEGIN_NAMESPACE_STD
namespace placeholders
{
__ph<1> _1;
__ph<2> _2;
__ph<3> _3;
__ph<4> _4;
__ph<5> _5;
__ph<6> _6;
__ph<7> _7;
__ph<8> _8;
__ph<9> _9;
__ph<10> _10;
const __ph<1> _1{};
const __ph<2> _2{};
const __ph<3> _3{};
const __ph<4> _4{};
const __ph<5> _5{};
const __ph<6> _6{};
const __ph<7> _7{};
const __ph<8> _8{};
const __ph<9> _9{};
const __ph<10> _10{};
} // placeholders

28
libcxx/test/std/utilities/function.objects/bind/func.bind/func.bind.place/placeholders.pass.cpp
View File

@ -10,10 +10,14 @@
// <functional>
// placeholders
// The placeholders are constexpr in C++17 and beyond.
// libc++ provides constexpr placeholders in C++11 and beyond.
#include <functional>
#include <type_traits>
#include "test_macros.h"
template <class T>
void
test(const T& t)
@ -28,6 +32,30 @@ test(const T& t)
static_assert(std::is_nothrow_move_constructible<T>::value, "");
}
#if TEST_STD_VER >= 11
constexpr decltype(std::placeholders::_1) default1{};
constexpr decltype(std::placeholders::_2) default2{};
constexpr decltype(std::placeholders::_3) default3{};
constexpr decltype(std::placeholders::_4) default4{};
constexpr decltype(std::placeholders::_5) default5{};
constexpr decltype(std::placeholders::_6) default6{};
constexpr decltype(std::placeholders::_7) default7{};
constexpr decltype(std::placeholders::_8) default8{};
constexpr decltype(std::placeholders::_9) default9{};
constexpr decltype(std::placeholders::_10) default10{};
constexpr decltype(std::placeholders::_1) cp1 = std::placeholders::_1;
constexpr decltype(std::placeholders::_2) cp2 = std::placeholders::_2;
constexpr decltype(std::placeholders::_3) cp3 = std::placeholders::_3;
constexpr decltype(std::placeholders::_4) cp4 = std::placeholders::_4;
constexpr decltype(std::placeholders::_5) cp5 = std::placeholders::_5;
constexpr decltype(std::placeholders::_6) cp6 = std::placeholders::_6;
constexpr decltype(std::placeholders::_7) cp7 = std::placeholders::_7;
constexpr decltype(std::placeholders::_8) cp8 = std::placeholders::_8;
constexpr decltype(std::placeholders::_9) cp9 = std::placeholders::_9;
constexpr decltype(std::placeholders::_10) cp10 = std::placeholders::_10;
#endif // TEST_STD_VER >= 11
int main()
{
test(std::placeholders::_1);

2
libcxx/www/cxx1z_status.html
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@ -161,7 +161,7 @@
<tr><td><a href="http://cplusplus.github.io/LWG/lwg-defects.html#2467">2467</td><td>is_always_equal has slightly inconsistent default</td><td>Lenexa</td><td>Complete</td></tr>
<tr><td><a href="http://cplusplus.github.io/LWG/lwg-defects.html#2470">2470</td><td>Allocator's destroy function should be allowed to fail to instantiate</td><td>Lenexa</td><td>Complete</td></tr>
<tr><td><a href="http://cplusplus.github.io/LWG/lwg-defects.html#2482">2482</td><td>[c.strings] Table 73 mentions nonexistent functions</td><td>Lenexa</td><td>Complete</td></tr>
<tr><td><a href="http://cplusplus.github.io/LWG/lwg-defects.html#2488">2488</td><td>Placeholders should be allowed and encouraged to be constexpr</td><td>Lenexa</td><td></td></tr>
<tr><td><a href="http://cplusplus.github.io/LWG/lwg-defects.html#2488">2488</td><td>Placeholders should be allowed and encouraged to be constexpr</td><td>Lenexa</td><td>Complete</td></tr>
<tr><td></td><td></td><td></td><td></td></tr>
<tr><td><a href="http://cplusplus.github.io/LWG/lwg-defects.html#1169">1169</a></td><td><tt>num_get</tt> not fully compatible with <tt>strto*</tt></td><td>Kona</td><td>Complete</td></tr>
<tr><td><a href="http://cplusplus.github.io/LWG/lwg-defects.html#2072">2072</a></td><td>Unclear wording about capacity of temporary buffers</td><td>Kona</td><td>Complete</td></tr>