Templates otl_value<T> / otl_compact_value<T,null> / std::optional<T>

Derivative containers based on otl_value<T> have built-in NULL indicator functionality, that is, which can carry over the NULL value from one operation to another.

Derivative data containers based on otl_compact_value<T,null> have NULL value encoded via the second template parameter, and carry over the NULL value from one operation to another.

For example, a value gets read from a SELECT statement into a otl_value<int> / otl_compact_value<int,-1> container, and, it is a NULL. Then, the same value gets written into an INSERT statement. The otl_value<int> / otl_compact_value<int,-1> container retains the NULL, and the NULL gets carried over from the SELECT into the INSERT.-1 represents NULL value is this example.

The otl_value<T> / otl_compact_value<T,null> classes can be activated with #define OTL_STL, or #define OTL_VALUE_TEMPLATE_ON.
 

template<class TData> 
class otl_value{
public:

TData v;

bool ind;

otl_value();

otl_value(const TData& var); 

otl_value(const otl_null var); 

otl_value<TData>& operator=
(const otl_value<TData>& var); 

otl_value<TData>& operator=
(const TData& var); 

otl_value<TData>& operator=
(const otl_null var); 

bool is_null(void) const; 

void set_null(void); 

void set_null(const bool null);

void set_non_null(void); 

}; // end of otl_value


template<class TData, const TData null_value> 
class otl_compact_value{
public:    
// null_value is a constant of the TData type that encodes NULL value for each instantiation of the template.

TData v;

otl_compact_value();

otl_compact_value(const TData& var);

otl_compact_value(const otl_null var);

otl_compact_value<TData,null_value>& operator=
(const otl_compact_value<TData,null_value>& var);

otl_compact_value<TData,null_value>& operator=
(const TData& var);

otl_compact_value<TData,null_value>& operator=
(const otl_null var); 

bool is_null(void) const;

void set_null(const bool null);

}; // end of otl_compact_value

template <class TData>
otl_stream& operator<<(otl_stream& s, const otl_value<TData>& var);

template <class TData>
otl_stream& operator>>(otl_stream& s, otl_value<TData>& var);

template <class TData, const TData null_value>
otl_stream& operator<<(otl_stream& s, const otl_compact_value<TData,null_value>& var);

template <class TData, const TData null_value>
otl_stream& operator>>(otl_stream& s, otl_compact_value<TData,null_value>& var);

Here's two more overloaded operator<< for writing otl_value<T> / otl_compact_value<T,null>'s to std::ostream:

template <class TData> std::ostream&
operator<<(std::ostream& s, const otl_value<TData>& var);

template <class TData, const TData null_value> std::ostream&
operator<<(std::ostream& s, const otl_compact_value<TData,null_value>& var);

As the name suggests, otl_compact_value<T,null> is more memory efficient compared with otl_value<T> (sizeof(otl_compact_value<T>) == sizeof(T), sizeof(otl_value<T>) == sizeof(T) + sizeof(bool) + padding). otl_compact_value<T,null> should be used when there is a magic value for type T that can represent NULL value in a given context. There may be many instantiations of otl_compact_value with the same T but with different null_values. otl_compact_value<T,null> can be used with non-integral types, C++11 compilers and higher, when otl_compact_value's second template non-type parameter has external linkage and operator==() defined for type T (first template parameter).

For more detail on otl_compact_value<T,null>, see code examples 98, 99, and 100. If you can't find a code example for your database type, it's easy to migrate the otl_compact_value based code from examples for other database types.

OTL can also be used with std::experimental::optional<T> / std::optional<T> and the following data types: int, unsigned, long, short,  float, double, otl_datetime (OTL date&time container), and std::string (STL string class, requires #define OTL_STL). , OTL defines operators>>/<< that work with the optional<T> templates as template member functions of the otl_stream class, when #define OTL_CPP_14_ON or #define OTL_CPP_17_ON is  auto or explicitly enabled, and when #define OTL_STREAM_WITH_STD_OPTIONAL_ON is enabled:

...

#if defined(OTL_STREAM_WITH_STD_OPTIONAL_ON)

template<typename TData, template <typename> class Optional> otl_stream &operator<<(const Optional<TData> &var);
template<typename TData, template <typename> class Optional> otl_stream &operator>>(Optional<TData> &var);

#endif

...
};

When frac_precision of otl_datetime is different from its default value of 0, and when std::optional<otl_datetime> is used to read values from an otl_stream, such variables should be initialized, for example:

  std::optional<otl_datetime> f(otl_datetime(2016,10,27,13,21,15,6,123456));

The reason is that OTL uses the frac_precision of an otl_datetime container that OTL writes into (a.k.a output variable) to format the output otl_datetime::fraction correctly.

OTL utilizes the buffer of a std::optional<T> variable (a.k.a. direct use, no temporaries, more efficient), if the variable has a value (!= std::nullopt). In the case of an empty (==std::nullopt) std::optional<T> variable, OTL uses std::move() when a move constructor is available for type T.

When OTL writes (operator <<) an optional<> variable and it has no value, OTL treats the absence of value as NULL and writes the NULL into the database. And when OTL reads (operator>>) NULL from the database, OTL sets the output optional<> variable to "no value", which can be checked with optional<T>::operator bool() or has_value() function.

For more detail on std::optional<T>, see code examples 98, 99, 100, and  204. If you can't find a code example for your database type, it's easy to migrate the otl_compact_value based code from examples for other database types.

Copyright © 1996-2023, Sergei Kuchin, email: skuchin@gmail.com, skuchin@gmail.com .

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