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Allow using polymorphic_downcast and polymorphic_cast in constexpr #30

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Merged
apolukhin merged 2 commits into from
Jun 27, 2025
Merged

Allow using polymorphic_downcast and polymorphic_cast in constexpr #30

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5b5faf7
Allow using polymorphic_downcast and polymorphic_cast in constexpr
apolukhin Jun 27, 2025
16a7491
one more test
apolukhin Jun 27, 2025
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6 changes: 3 additions & 3 deletions doc/conversion.qbk
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Original file line number Diff line number Diff line change
Expand Up @@ -173,17 +173,17 @@ This includes C++ built-in pointers, `std::shared_ptr`,
// Throws: std::bad_cast if ( dynamic_cast<Derived>(x) == 0 )
// Returns: dynamic_cast<Derived>(x)
template <class Derived, class Base>
inline Derived polymorphic_cast(Base* x);
constexpr Derived polymorphic_cast(Base* x);

// Effects: assert( dynamic_cast<Derived>(x) == x );
// Returns: static_cast<Derived>(x)
template <class Derived, class Base>
inline Derived polymorphic_downcast(Base* x);
constexpr Derived polymorphic_downcast(Base* x);

// Effects: assert( dynamic_cast<Derived>(&x) == &x );
// Returns: static_cast<Derived>(x)
template <class Derived, class Base>
inline Derived polymorphic_downcast(Base& x);
constexpr Derived polymorphic_downcast(Base& x);

// Throws: std::bad_cast if ( dynamic_pointer_cast<Derived>(x) == 0 )
// Returns: dynamic_pointer_cast<Derived>(x)
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14 changes: 11 additions & 3 deletions include/boost/polymorphic_cast.hpp
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Expand Up @@ -62,6 +62,12 @@
# include <typeinfo>
# include <type_traits>

#if defined(__cpp_constexpr) && __cpp_constexpr >= 201907L
#define BOOST_CONVERSION_IMPL_CONSTEXPR_DYN_CAST constexpr
#else
#define BOOST_CONVERSION_IMPL_CONSTEXPR_DYN_CAST inline
#endif

namespace boost
{
// See the documentation for descriptions of how to choose between
Expand All @@ -74,7 +80,7 @@ namespace boost
// section 15.8 exercise 1, page 425.

template <class Target, class Source>
inline Target polymorphic_cast(Source* x)
BOOST_CONVERSION_IMPL_CONSTEXPR_DYN_CAST Target polymorphic_cast(Source* x)
{
Target tmp = dynamic_cast<Target>(x);
if ( tmp == 0 ) boost::throw_exception( std::bad_cast() );
Expand All @@ -93,7 +99,7 @@ namespace boost
// Contributed by Dave Abrahams

template <class Target, class Source>
inline Target polymorphic_downcast(Source* x)
BOOST_CONVERSION_IMPL_CONSTEXPR_DYN_CAST Target polymorphic_downcast(Source* x)
{
BOOST_ASSERT( dynamic_cast<Target>(x) == x ); // detect logic error
return static_cast<Target>(x);
Expand All @@ -109,7 +115,7 @@ namespace boost
// Contributed by Julien Delacroix

template <class Target, class Source>
inline typename std::enable_if<
BOOST_CONVERSION_IMPL_CONSTEXPR_DYN_CAST typename std::enable_if<
std::is_reference<Target>::value, Target
>::type polymorphic_downcast(Source& x)
{
Expand All @@ -121,4 +127,6 @@ namespace boost

} // namespace boost

#undef BOOST_CONVERSION_IMPL_CONSTEXPR_DYN_CAST

#endif // BOOST_POLYMORPHIC_CAST_HPP
42 changes: 42 additions & 0 deletions test/cast_test.cpp
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Expand Up @@ -38,6 +38,48 @@ namespace
};
}

constexpr bool compile_time_polymorphic_cast_check() {
#if defined(__cpp_constexpr) && __cpp_constexpr >= 201907L
Derived derived;
Base* base = &derived;
return polymorphic_cast<Derived*>(base) != nullptr;
#endif
return true;
}

static_assert(
compile_time_polymorphic_cast_check(),
"polymorphic_cast does not work at compile time"
);

constexpr bool compile_time_polymorphic_downcast_check() {
#if defined(__cpp_constexpr) && __cpp_constexpr >= 201907L
Derived derived;
Base* base = &derived;
return polymorphic_downcast<Derived*>(base) != nullptr;
#endif
return true;
}

static_assert(
compile_time_polymorphic_downcast_check(),
"polymorphic_downcast does not work at compile time"
);

constexpr bool compile_time_polymorphic_downcast2_check() {
#if defined(__cpp_constexpr) && __cpp_constexpr >= 201907L
Derived derived;
Base& base = derived;
Derived& derived_again = polymorphic_downcast<Derived&>(base);
(void)derived_again;
#endif
return true;
}

static_assert(
compile_time_polymorphic_downcast2_check(),
"polymorphic_downcast does not work at compile time"
);

int main( int argc, char * argv[] )
{
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