您不需要手动检查类型是否存在。如果它们不存在,SFINAE 将使您的概念默默返回false
反正。所以:
template< typename U, typename Tin, typename Tout>
concept MyConditions =
std::is_integral_v<typename T::value_type> &&
std::is_floating_point_v<typename U::value_type> &&
std::is_floating_point_v<typename Tout::value_type>;
但如果您想显式检查类型,语法如下:
template< typename U, typename Tin, typename Tout>
concept MyConditions =
requires
{
typename U::value_type;
typename Tin::value_type;
typename Tout::value_type;
} &&
std::is_integral_v<typename T::value_type> &&
std::is_floating_point_v<typename U::value_type> &&
std::is_floating_point_v<typename Tout::value_type>;
您还可以将所有条件移至requires
:
template< typename U, typename Tin, typename Tout>
concept MyConditions =
requires
{
typename U::value_type;
typename Tin::value_type;
typename Tout::value_type;
requires std::is_integral_v<typename T::value_type>;
requires std::is_floating_point_v<typename U::value_type>;
requires std::is_floating_point_v<typename Tout::value_type>;
};
此外,您应该更喜欢标准概念而不是旧特征:
template< typename U, typename Tin, typename Tout>
concept MyConditions =
requires
{
typename U::value_type;
typename Tin::value_type;
typename Tout::value_type;
requires std::integral<typename T::value_type>;
requires std::floating_point<typename U::value_type>;
requires std::floating_point<typename Tout::value_type>;
};