@@ -439,7 +439,7 @@ namespace __variant
constexpr bool
_M_valid() const noexcept
{
- if constexpr ((is_scalar_v<_Types> && ...))
+ if constexpr ((is_trivially_copyable_v<_Types> && ...))
return true;
return this->_M_index != __index_type(variant_npos);
}
@@ -1185,6 +1185,23 @@ namespace __variant
{
static_assert(_Np < sizeof...(_Types),
"The index should be in [0, number of alternatives)");
+
+ using type = variant_alternative_t<_Np, variant>;
+ // If constructing the value can throw but move assigning it can't,
+ // construct in a temporary and then move assign from it. This gives
+ // the strong exception safety guarantee, ensuring we never become
+ // valueless.
+ if constexpr (is_trivially_copyable_v<type>
+ && !is_nothrow_constructible_v<type, _Args...>)
+ {
+ // If move assignment cannot throw then we can provide the
+ // strong exception safety guarantee, and never become valueless.
+ variant __tmp(in_place_index<_Np>,
+ std::forward<_Args>(__args)...);
+ *this = std::move(__tmp);
+ return std::get<_Np>(*this);
+ }
+
this->~variant();
__try
{
@@ -1208,6 +1225,20 @@ namespace __variant
{
static_assert(_Np < sizeof...(_Types),
"The index should be in [0, number of alternatives)");
+
+ using type = variant_alternative_t<_Np, variant>;
+ if constexpr (is_trivially_copyable_v<type>
+ && !is_nothrow_constructible_v<type, initializer_list<_Up>,
+ _Args...>)
+ {
+ // If move assignment cannot throw then we can provide the
+ // strong exception safety guarantee, and never become valueless.
+ variant __tmp(in_place_index<_Np>, __il,
+ std::forward<_Args>(__args)...);
+ *this = std::move(__tmp);
+ return std::get<_Np>(*this);
+ }
+
this->~variant();
__try
{
new file mode 100644
@@ -0,0 +1,71 @@
+// Copyright (C) 2019 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library. This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3. If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++17" }
+// { dg-do run { target c++17 } }
+
+#include <variant>
+#include <testsuite_hooks.h>
+
+void
+test01()
+{
+ struct ThrowingConversion {
+ operator int() { throw 0; }
+ };
+
+ std::variant<float, char, int> v{'a'};
+ try
+ {
+ ThrowingConversion x;
+ v.emplace<2>(x);
+ VERIFY(false);
+ }
+ catch (int)
+ {
+ VERIFY( !v.valueless_by_exception() );
+ VERIFY( v.index() == 1 );
+ VERIFY( std::get<1>(v) == 'a' );
+ }
+}
+
+void
+test02()
+{
+ struct ThrowingConstructor {
+ ThrowingConstructor(std::initializer_list<int>, char) { throw 1; }
+ };
+
+ std::variant<float, char, ThrowingConstructor> v{'a'};
+ try
+ {
+ v.emplace<2>({1, 2, 3}, '4');
+ VERIFY(false);
+ }
+ catch (int)
+ {
+ VERIFY( !v.valueless_by_exception() );
+ VERIFY( v.index() == 1 );
+ VERIFY( std::get<1>(v) == 'a' );
+ }
+}
+
+int
+main()
+{
+ test01();
+}
@@ -354,7 +354,7 @@ void test_hash()
{
struct A
{
- operator int()
+ operator string()
{
throw nullptr;
}
@@ -362,7 +362,7 @@ void test_hash()
variant<int, string> v;
try
{
- v.emplace<0>(A{});
+ v.emplace<1>(A{});
}
catch (nullptr_t)
{
The previous patch for PR 87431 assumed that initialing a scalar type could not throw, but it can obtain its value via a conversion operator, which could throw. This meant the variant could get into a valueless state, but the valueless_by_exception() member function would always return false. This patch fixes it by changing the emplace members to have strong exception safety when initializing a contained value of trivially copyable type. The _M_valid() member gets a corresponding change to always return true for trivially copyable types, not just scalar types. Strong exception safety (i.e. never becoming valueless) is achieved by only replacing the current contained value once any potentially throwing operations have completed. If constructing the new contained value can throw then a new std::variant object is constructed to hold it, and then move-assigned to *this (which won't throw). PR libstdc++/87431 * include/std/variant (_Variant_storage<true, _Types...>::_M_valid): Check is_trivially_copyable instead of is_scalar. (variant::emplace<N, Args>(Args&&...)): If construction of the new contained value can throw and its type is trivially copyable then construct into a temporary variant and move from it, to provide the strong exception safety guarantee. (variant::emplace<N, U, Args>(initializer_list<U>, Args&&...)): Likewise. * testsuite/20_util/variant/87431.cc: New test. * testsuite/20_util/variant/run.cc: Adjust test so that throwing conversion causes valueless state. Tested powerpc64-linux, committed to trunk. commit 54cf49b46d1517752290f9e2c39604f7d40784c6 Author: Jonathan Wakely <jwakely@redhat.com> Date: Fri Jan 4 18:07:50 2019 +0000 PR libstdc++/87431 fix regression introduced by r264574 The previous patch for PR 87431 assumed that initialing a scalar type could not throw, but it can obtain its value via a conversion operator, which could throw. This meant the variant could get into a valueless state, but the valueless_by_exception() member function would always return false. This patch fixes it by changing the emplace members to have strong exception safety when initializing a contained value of trivially copyable type. The _M_valid() member gets a corresponding change to always return true for trivially copyable types, not just scalar types. Strong exception safety (i.e. never becoming valueless) is achieved by only replacing the current contained value once any potentially throwing operations have completed. If constructing the new contained value can throw then a new std::variant object is constructed to hold it, and then move-assigned to *this (which won't throw). PR libstdc++/87431 * include/std/variant (_Variant_storage<true, _Types...>::_M_valid): Check is_trivially_copyable instead of is_scalar. (variant::emplace<N, Args>(Args&&...)): If construction of the new contained value can throw and its type is trivially copyable then construct into a temporary variant and move from it, to provide the strong exception safety guarantee. (variant::emplace<N, U, Args>(initializer_list<U>, Args&&...)): Likewise. * testsuite/20_util/variant/87431.cc: New test. * testsuite/20_util/variant/run.cc: Adjust test so that throwing conversion causes valueless state.