qt-everywhere-src-5.14.1/qtlocation/src/3rdparty/mapbox-gl-native/include/mbgl/style/property_value.hpp

#pragma once

#include <mbgl/util/variant.hpp>
#include <mbgl/style/undefined.hpp>
#include <mbgl/style/property_expression.hpp>

namespace mbgl {
namespace style {

template <class T>
class PropertyValue {
private:
    using Value = variant<Undefined, T, PropertyExpression<T>>;
    Value value;

    friend bool operator==(const PropertyValue& lhs, const PropertyValue& rhs) {
        return lhs.value == rhs.value;
    }

    friend bool operator!=(const PropertyValue& lhs, const PropertyValue& rhs) {
        return !(lhs == rhs);
    }

public:
    PropertyValue()
        : value() {}

    PropertyValue(T constant)
        : value(constant) {}

    PropertyValue(PropertyExpression<T> expression)
        : value(expression) {
        assert(expression.isFeatureConstant());
    }

    bool isUndefined()      const { return value.which() == 0; }
    bool isConstant()       const { return value.which() == 1; }
    bool isExpression()     const { return value.which() == 2; }
    bool isDataDriven()     const { return false; }

    const                    T & asConstant()   const { return value.template get<                   T >(); }
    const PropertyExpression<T>& asExpression() const { return value.template get<PropertyExpression<T>>(); }

    template <typename Evaluator>
    auto evaluate(const Evaluator& evaluator, TimePoint = {}) const {
        return Value::visit(value, evaluator);
    }

    bool hasDataDrivenPropertyDifference(const PropertyValue<T>&) const {
        return false;
    }
};

} // namespace style
} // namespace mbgl