BloombergLP::bslstl::Variant_ConvertsWithoutNarrowing

Synopsis

Declared in <bslstl_variant.h>

template<
    class t_DEST,
    class t_SOURCE,
    class = void>
struct Variant_ConvertsWithoutNarrowing
    : bsl::false_type

Base Classes

Name	Description
`bsl::false_type`	Metafunction representing the boolean constant `false`.

Specializations

Name Description

Variant_ConvertsWithoutNarrowing<t_DEST, t_SOURCE, bsl::void_t<decltype(Variant_ArrayHelper<t_DEST>{{std::declval<t_SOURCE>()}})>> This component-private metafunction is derived from bsl::true_type if (template parameter) t_SOURCE can be converted to (template parameter) t_DEST without narrowing, and bsl::false_type otherwise. A conversion from pointer or pointer-to-member type to cv bool is considered narrowing even if the compiler does not implement P1957R2 (which was adopted as a DR); however, on compilers that do not implement P1957R2, we do not have the ability to check whether a user-defined conversion sequence to cv bool would use a narrowing standard conversion, so on those compilers, we permit conversion to a t_DEST that is cv bool only if t_SOURCE is also cv bool, and not when t_SOURCE is a class type. This behavior is not expected to pose a problem for users migrating from bdlb::Variant, because that class does not support implicit conversions from an argument type to an alternative type.

Name	Description
`Variant_ConvertsWithoutNarrowing<t_DEST, t_SOURCE, bsl::void_t<decltype(Variant_ArrayHelper<t_DEST>{{std::declval<t_SOURCE>()}})>>`	This component-private metafunction is derived from `bsl::true_type` if (template parameter) `t_SOURCE` can be converted to (template parameter) `t_DEST` without narrowing, and `bsl::false_type` otherwise. A conversion from pointer or pointer-to-member type to cv `bool` is considered narrowing even if the compiler does not implement P1957R2 (which was adopted as a DR); however, on compilers that do not implement P1957R2, we do not have the ability to check whether a user-defined conversion sequence to cv `bool` would use a narrowing standard conversion, so on those compilers, we permit conversion to a `t_DEST` that is cv `bool` only if `t_SOURCE` is also cv `bool`, and not when `t_SOURCE` is a class type. This behavior is not expected to pose a problem for users migrating from `bdlb::Variant`, because that class does not support implicit conversions from an argument type to an alternative type.