RWDBValue
In the DB Interface Module, class RWDBValue is the means of integrating data in the value layer with the pointer layer. RWDBValue automatically converts between compatible C++ and structured types, allowing the value layer to be type safe. An RWDBValue-based accessor and mutator are provided on RWDBTBuffer<T>, giving the Open SQL interface the flexibility needed to support the value layer. RWDBValue also adds null/not null semantics to the primitive types.
In application programs, RWDBValue is especially useful when working with results where the type of data is not known. Programs can read data directly into RWDBValue instances, and then interrogate the instance to find out what kind of data was placed in it. RWDBValue instances can be held in collections and passed without knowing what type of data they are holding. Conversion methods on RWDBValue allow applications to handle unknown data in a robust, predictable manner.
You will find a complete entry for RWDBValue in the SourcePro API Reference Guide, but its main components are listed here:
An enumerated type, ValueType, which codes the datatype of the stored value. The possible ValueTypes are NoType, Tiny, UnsignedTiny, Char, UnsignedChar, Int, UnsignedInt, Short, UnsignedShort, Long, UnsignedLong, LongLong, UnsignedLongLong, Float, Double, LongDouble, String, Blob, Date, DateTime,TimeTuple, TimeTupleOffset, Duration, Decimal, UString, WString, and MBString. To avoid polluting the global name space, all enums are scoped within classes. To refer to a member of the enumeration outside the scope of RWDBValue, use the scoping operator ::, as in RWDBValue::String, for example.A friend, rwdbNull, which is of type RWDBValueManip, and represents a literal NULL value. There is a constructor and an assignment operator that allow RWDBValue instances to be formed from rwdbNulls and other RWDBValueManips.A C++ union that can store a Long, Unsigned Long, Long Long, Unsigned Long Long, Double, Long Double, RWCString, RWDBMBString, RWWString, RWBasicUString, RWDecimalPortable, RWDBDuration, or RWDBBlob.Member functions type()and isNull()to access the value's datatype and NULL attribute, respectively. Note that isNull() cannot be used to determine if a value extracted from an RWDBReader is NULL. Please refer to Reading Tables: Class RWDBReader and the entries in the SourcePro API Reference Guide for RWDBNullIndicator and RWDBReader for more information.Constructors and assignment operators that allow an RWDBValue to be constructed or copied from any C++ primitive or from any structured type of the DB Interface Module.The member function canConvert(ValueType), which detects whether or not a given value can be converted to a specified type.A collection of conversion routines such as asInt(), asDouble(), asString(), and so on, which convert a given value to the type implied by the routine name.The member function typeString() returns the datatype of self in the form of a string.Member functions isEqual(), compareTo(), binaryStoreSize(), saveGuts(), and restoreGuts(), which are required to make RWDBValue persistable, as defined in the Essential Tools Module. RWDBValue is derived from RWCollectable.As mentioned earlier, RWDBValue is used internally to integrate the value and pointer layers of the DB Interface Module. Whenever data arrives from a database, routines in the database layer interrogate the database API for its type, and store the data into RWDBTBuffer<T>, where T is the primitive or structured type of the DB Interface Module best able to hold the data. RWDBTBuffer<T> provides an accessor that automatically converts data elements into RWDBValue instances for the value layer. Whenever the value layer requests data from the pointer layer, RWDBValue’s canConvert() routine is consulted to determine whether or not the conversion can be made. If not, an error condition arises. Otherwise, the appropriate as<Type>() routine is invoked to convert to the requested type.
