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Fix PR19460 - std::ios is convertible to int. 

std::basic_ios has an operator bool(). In C++11 and later
it is explicit, and only allows contextual implicit conversions.

However explicit isn't available in C++03 which causes std::istream (et al)
to have an implicit conversion to int. This can easily cause ambiguities
when calling operator<< and operator>>.

This patch uses a "bool-like" type in C++03 to work around this. The
"bool-like" type is an arbitrary pointer to member function type. It
will not convert to either int or void*, but will convert to bool.

llvm-svn: 290750
This commit is contained in:
Eric Fiselier 2016-12-30 12:44:58 +00:00
parent a8d45de6ce
commit 2131a71c05
2 changed files with 22 additions and 2 deletions
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17
libcxx/include/ios
View File

@ -585,9 +585,22 @@ public:
typedef typename traits_type::pos_type pos_type;
typedef typename traits_type::off_type off_type;
#if defined(_LIBCPP_CXX03_LANG)
private:
struct __bool_tag {};
typedef void (basic_ios::*_BoolType)(__bool_tag) const;
void __true_value(__bool_tag) const {}
public:
_LIBCPP_ALWAYS_INLINE
_LIBCPP_EXPLICIT
operator bool() const {return !fail();}
operator _BoolType() const {
return !fail() ? &basic_ios::__true_value : nullptr;
}
#else
_LIBCPP_ALWAYS_INLINE
_LIBCPP_EXPLICIT operator bool() const {return !fail();}
#endif
_LIBCPP_ALWAYS_INLINE bool operator!() const {return fail();}
_LIBCPP_ALWAYS_INLINE iostate rdstate() const {return ios_base::rdstate();}
_LIBCPP_ALWAYS_INLINE void clear(iostate __state = goodbit) {ios_base::clear(__state);}

7
libcxx/test/std/input.output/iostreams.base/ios/iostate.flags/bool.pass.cpp
View File

@ -14,6 +14,7 @@
// operator unspecified-bool-type() const;
#include <ios>
#include <type_traits>
#include <cassert>
int main()
@ -22,4 +23,10 @@ int main()
assert(static_cast<bool>(ios) == !ios.fail());
ios.setstate(std::ios::failbit);
assert(static_cast<bool>(ios) == !ios.fail());
static_assert((!std::is_convertible<std::ios, void*>::value), "");
static_assert((!std::is_convertible<std::ios, int>::value), "");
static_assert((!std::is_convertible<std::ios const&, int>::value), "");
#if TEST_STD_VER >= 11
static_assert((!std::is_convertible<std::ios, bool>::value), "");
#endif
}