[英]How to support implicit conversion from a Variant type, e.g. from int to unsigned long?
我試圖讓我的Variant
類型,一個圍繞 c++17 的std::variant
的包裝器,在適當的地方隱式轉換類型。 例如,將char
轉換為std::string
,或將int
轉換為unsigned long
。 這是我的代碼:
#include <variant>
using variant_t = std::variant<
std::monostate,
std::string, bool, std::int32_t,
std::uint32_t, std::int64_t, std::uint64_t,
float, double, char, unsigned char,
std::vector<double>>;
class Variant : public variant_t {
public:
using variant::variant;
enum TypeId {
EMPTY = 0, // std::monostate. Empty is default when variant instantiated with nothing
STRING = 1,
BOOL = 2,
INT32 = 3,
UINT32 = 4,
INT64 = 5,
UINT64 = 6,
FLOAT = 7,
DOUBLE = 8,
CHAR = 9,
UCHAR = 10,
DOUBLEVECTOR = 11
};
TypeId type() const {
return (Variant::TypeId) index();
}
template<class VariantType>
VariantType get() const {
return std::get<VariantType>(*this);
}
};
我想要做的是:
TEST(VariantTests, HowToConvertIntToULongWithoutManualCast) {
Variant v(11);
ASSERT_EQ(v.type(), Variant::TypeId::INT32); // (pass, v is an int)
unsigned long toUnsignedLong = v; // error
long toLong = v; // error
// and any other conversions from int that make sense
}
如何修改我的Variant
以支持隱式類型轉換?
根據評論,我還需要考慮不兼容的對以及兼容的對,例如以下將失敗。
Variant v(12); // variant containing an int
std::string x = v; // should error, int to string incompatible
這些方面的東西:
template <typename T>
struct Visitor {
template <typename U>
std::enable_if_t<std::is_convertible_v<U, T>, T> operator() (U&& val) {
return val;
}
template <typename U>
std::enable_if_t<!std::is_convertible_v<U, T>, T> operator() (U&& val) {
throw std::bad_variant_access{};
}
};
template<class VariantType>
VariantType get() const {
return std::visit(Visitor<VariantType>{}, static_cast<const variant_t&>(*this));
}
對於基於operator T ()
解決方案(但以std::variant
作為成員,不繼承)
template <typename T>
operator T () const
{
return std::visit(
[](auto const & val)
{ if constexpr ( std::is_convertible_v<decltype(val), T> )
return T(val);
else
{ throw std::bad_variant_access{}; return T{}; } }, var);
}
下面是一個完整的編譯示例
#include <vector>
#include <string>
#include <variant>
#include <cstdint>
#include <type_traits>
using variant_t = std::variant<
std::monostate,
std::string, bool, std::int32_t,
std::uint32_t, std::int64_t, std::uint64_t,
float, double, char, unsigned char,
std::vector<double>>;
class Variant
{
public:
variant_t var_;
explicit Variant (variant_t var)
: var_(std::move(var)){}
enum TypeId
{
EMPTY = 0,
STRING = 1,
BOOL = 2,
INT32 = 3,
UINT32 = 4,
INT64 = 5,
UINT64 = 6,
FLOAT = 7,
DOUBLE = 8,
CHAR = 9,
UCHAR = 10,
DOUBLEVECTOR = 11
};
TypeId type () const
{ return (Variant::TypeId) var_.index(); }
template <typename VariantType>
VariantType get () const
{ return std::get<VariantType>(var_); }
template <typename T>
operator T () const
{
return std::visit(
[](auto const & val)
{ if constexpr ( std::is_convertible_v<decltype(val), T> )
return T(val);
else
{ throw std::bad_variant_access{}; return T{}; } }, var_);
}
};
int main()
{
Variant v{12};
long l = v; // compile and works run-time
std::string s = v; // compile and throw run-time
}
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