cast.h

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// SPDX-License-Identifier: GPL-2.0-or-later
#ifndef INKSCAPE_UTIL_CAST_H
#define INKSCAPE_UTIL_CAST_H
 
#include <type_traits>
 
/*
 * In order to use this system with a class hierarchy, specialize the following templates for
 * every member of the hierarchy. The only requirement is that
 *
 *     first_tag<T> <= first_tag<S> <= last_tag<T>
 *
 * exactly when S is a derived class of T. Then add to each class the line of boilerplate
 *
 *     int tag() const override { return tag_of<decltype(*this)>; }
 */
template <typename T> inline constexpr int first_tag = std::enable_if<!sizeof(T), void>::value;
template <typename T> inline constexpr int last_tag  = std::enable_if<!sizeof(T), void>::value;
 
template <typename T> inline constexpr int tag_of = first_tag<std::remove_cv_t<std::remove_reference_t<T>>>;
 
template<typename T, typename S>
bool is(S const *s)
{
    if (!s) return false;
    if constexpr (std::is_base_of_v<T, S>) {
        static_assert(!sizeof(T), "check is always true");
        return true;
    } else if constexpr (std::is_base_of_v<S, T>) {
        auto const s_tag = s->tag();
        return first_tag<T> <= s_tag && s_tag <= last_tag<T>;
    } else {
        static_assert(!sizeof(T), "check is always false");
        return false;
    }
}
 
template<typename T, typename S>
auto cast_unsafe(S *s)
{
    return static_cast<T*>(s);
}
 
template<typename T, typename S>
auto cast_unsafe(S const *s)
{
    return static_cast<T const*>(s);
}
 
template<typename T, typename S>
auto cast(S *s)
{
    if constexpr (std::is_base_of_v<T, S>) {
        // Removed static assert; it complicates template "collect_items"
        // static_assert(!sizeof(T), "cast is unnecessary");
        return cast_unsafe<T>(s);
    } else if constexpr (std::is_base_of_v<S, T>) {
        return is<T>(s) ? cast_unsafe<T>(s) : nullptr;
    } else {
        static_assert(!sizeof(T), "cast is impossible");
        return nullptr;
    }
}
 
#endif // INKSCAPE_UTIL_CAST_H
 
/*
  Local Variables:
  mode:c++
  c-file-style:"stroustrup"
  c-file-offsets:((innamespace . 0)(inline-open . 0)(case-label . +))
  indent-tabs-mode:nil
  fill-column:99
  End:
*/
// vim: filetype=cpp:expandtab:shiftwidth=4:tabstop=8:softtabstop=4:fileencoding=utf-8:textwidth=99 :