| Member Functions | |
| operator()() |
#include <rw/db/func.h>
RWDBCritFormDefinition myFunc("myFormat");
RWDBCritFormDefinition is a base class for a family of classes which provide an extensible mechanism that an application can use to define functional notation for RWDBCriterion. RWDBCritFormDefinition allows a programmer to create an object that will translate into an RWDBCriterion and eventually be expanded into an SQL string. The object will allow up to four expressions to be embedded into the resultant string.
An RWDBCritFormDefinition 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 in this way may subsequently be referenced in the same way that predefined functions like rwdbMax and rwdbAvg are referenced.
When an RWDBCritFormDefinition object is expanded by the asString() method of an RWDBCriterion, 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 RWDBCritFuncDef{n} classes.
Imagine that a database vendor's SQL variant provides the boolean function VALID_NAME(string), which can be used in a WHERE clause. DBTools.h++ has no direct support for this function, yet we would like to use it without losing the benefits of high-level constructs for data manipulation. To do so, we can use a function which we define ourselves:
RWDBCritFormDefinition validName("VALID_NAME(%0)");
RWDBTable potentialNames = myDbase.table("potentialNames");
RWDBColumn name = potentialNames["name"];
RWDBSelector usableNames = potentialNames.selector();
usableNames << name;
usableNames.where(validName(name));
Note that the number of arguments actually passed to validName is not checked. Compare this example with the RWDBCritFuncDef{n} example to see how such checking can be enabled.
RWDBExprFormDefinition provides a parallel mechanism for defining an RWDBExpr, rather than an RWDBCriterion.
RWDBCritFuncDef0, RWDBCritFuncDef1, RWDBCritFuncDef2, RWDBCritFuncDef3, and RWDBCritFuncDef4 are derived from RWDBCritFormDefinition. They provide a simplified interface for functions requiring 0-4 arguments, respectively, and do not require positional placeholders.
See RWDBCriterion for details on SQL WHERE clauses in DBTools.h++.
RWDBCritFormDefinition();
The default constructor creates an empty RWDBCritFormDefinition.
RWDBCritFormDefinition(const RWCString& formatString);
Creates an RWDBCritFormDefinition with the given formatString. Objects instantiated in this way may subsequently be referenced as though they were predefined SQL functions.
virtual RWDBCriterion operator() ();
virtual RWDBCriterion operator() (const RWDBExpr& expr0);
virtual RWDBCriterion operator() (const RWDBExpr& expr0,const RWDBExpr& expr1);
virtual RWDBCriterion
operator() (const RWDBExpr& expr0,const RWDBExpr& expr1,
const RWDBExpr& expr2);
virtual RWDBCriterion
operator() (const RWDBExpr& expr0, const RWDBExpr& expr1,
const RWDBExpr& expr2, const RWDBExpr& expr3);
Returns an RWDBCriterion 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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