ClickHouse/libs/libmysqlxx/include/mysqlxx/Null.h

#pragma once

#include <mysqlxx/Exception.h>


namespace mysqlxx
{


struct NullType {};
const NullType null = {};


/** Класс для NULL-able типов.
  * Использование:
  *        mysqlxx::Null<int> x = mysqlxx::null;
  *        std::cout << (x.isNull() ? "Ok." : "Fail.") << std::endl;
  *        x = 10;
  */
template <typename T>
class Null
{
public:
    T data;
    bool is_null;

    Null() : is_null(true) {}
    Null(NullType data) : is_null(true) {}
    explicit Null(const T & data_) : data(data_), is_null(false) {}

    operator T & ()
    {
        if (is_null)
            throw Exception("Value is NULL");
        return data;
    }

    operator const T & () const
    {
        if (is_null)
            throw Exception("Value is NULL");
        return data;
    }

    Null<T> & operator= (const T & data_) { is_null = false; data = data_; return *this; }
    Null<T> & operator= (const Null<T> & other) { is_null = other.is_null; data = other.data; return *this; }
    Null<T> & operator= (const NullType other) { is_null = true; data = T(); return *this; }

    bool isNull() const { return is_null; }

    bool operator< (const Null<T> & other) const
    {
        return is_null < other.is_null
            || (is_null == other.is_null && data < other.data);
    }

    bool operator< (const NullType other) const { return false; }

    bool operator== (const Null<T> & other) const
    {
        return is_null == other.is_null && data == other.data;
    }

    bool operator== (const T & other) const
    {
        return !is_null && data == other;
    }

    bool operator== (const NullType other) const { return is_null; }

    bool operator!= (const Null<T> & other) const
    {
        return !(*this == other);
    }

    bool operator!= (const NullType other) const { return !is_null; }

    bool operator!= (const T & other) const
    {
        return is_null || data != other;
    }
};


template<typename T>
T getValueFromNull(const Null<T> & maybe)
{
    if (maybe.isNull())
        return {};
    return maybe;
}

}