RWDBExprFormDefinition

Module:  DB Interface Module   Group:  Expression Classes

Does not inherit

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operator()()

RWDBExprFormDefinition()

#include <rw/db/func.h>

RWDBExprFormDefinition myFunc("myFormat");

RWDBExprFormDefinition is a base class for a family of classes that provide an extensible mechanism for an application to define functional notation for RWDBExpr. RWDBExprFormDefinition allows a programmer to create an object that will translate into an RWDBExpr and eventually be expanded into an SQL string. The object will allow up to four expressions to be embedded into the resultant string.

An RWDBExprFormDefinition object is instantiated with a format string. The format string may contain placeholders for up to four positional parameters, which are labeled %0 through %3. An object defined this way may subsequently be referenced like predefined functions, such as rwdbMax and rwdbAvg.

When an RWDBExprFormDefinition object is expanded by an asString() method of RWDBExpr, each instance of %0 is replaced with the first actual argument, each instance of %1 is replaced with the second actual argument, and so on. There can be multiple instances of a placeholder in the format string, and the placeholders may occur in any order. This allows great flexibility in defining functions, at the cost of losing compile-time checking on the number of arguments actually passed. To enable compile-time checking on the number of actual arguments, at the cost of some flexibility, use the RWDBExprFuncDef{n} classes.

Imagine that a database vendor's SQL variant provides the function DECRYPT(string, length, key). The DB Interface Module has no direct support for this function, yet we would like to use it without losing the benefits of its high-level constructs for data manipulation. To do so, we can use a function which we define ourselves:

RWDBExprFormDefinition decrypt("DECRYPT(%0, %1, %2)");
RWCString key = getMyKey();
RWDBTable users = myDbase.table("users");
RWDBColumn passWord = users["passWord"];
RWDBSelector select = users.selector();
select << decrypt(passWord, rwdbCharLength(passWord), key);

Note that the number of arguments actually passed to decrypt is not checked. Compare this example with the RWDBExprFuncDef{n} example to see how such checking can be enabled.

RWDBCritFormDefinition provides a parallel mechanism for defining an RWDBCriterion, rather than an RWDBExpr.

RWDBExprFuncDef0, RWDBExprFuncDef1, RWDBExprFuncDef2, RWDBExprFuncDef3, and RWDBExprFuncDef4 are derived from RWDBExprFormDefinition. They provide a simplified interface for functions requiring 0 - 4 arguments, respectively, and do not require positional placeholders.

See RWDBExpr for details on SQL expressions in the DB Interface Module, as well as for a complete listing of predefined SQL functions.

RWDBExprFormDefinition();

Creates an empty RWDBExprFormDefinition. Provided only for convenience.

RWDBExprFormDefinition(const RWCString& formatString);

Creates an RWDBExprFormDefinition with the given formatString. Objects instantiated this way may subsequently be referenced as though they were predefined SQL functions.

virtual RWDBExpr 
operator() ();
virtual RWDBExpr 
operator()(const RWDBExpr& expr0);
virtual RWDBExpr 
operator()(const RWDBExpr& expr0, const RWDBExpr& expr1);
virtual RWDBExpr 
operator()(const RWDBExpr& expr0, const RWDBExpr& expr1, 
           const RWDBExpr& expr2);
virtual RWDBExpr 
operator()(const RWDBExpr& expr0, const RWDBExpr& expr1, 
           const RWDBExpr& expr2, const RWDBExpr& expr3);

Returns an RWDBExpr with the ability to replace any instance of %0 in self's function definition with expr0, %1 with expr1, and so on.

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