Method Invocation

Instance Method Invocation

OQL allows one to call a instance method with or without parameters. The method can be written in C++ or in OQL. As in C++, method calls use a combination of the path expression operator and the function call operator.

As in C++ or Java, methods can be overloaded: that means that one can have differents methods with the same name and a different signature or differents methods with the same name and the same signature in a class hierarchy. The choice of the method to invoke is done at evaluation time not at compile time. For instance let two methods Person Person::f(in int, in int) and int Person::f(in float, in string), the method to be invoked in the expression p->f(x, y) is decided at evaluation time according to the true types of x and y:

p := first(select Person);

x := 1; y := 2;

p->f(x, y); // X::f(in int, in int) is invoked
p->f(x, y)->name; // this is valid because p->f(x, y) returns a Person

x := 1.3; y := "hello";

p->f(x, y); // X::f(in float, in string) is invoked
p->f(x, y)->name; // this is not valid because p->f(x, y) returns an integer

A major contribution of object orientation is the possibility of manipulating polymorphic objects and thanks to the late binding mechanism to carry out generic actions on the elements of these objects.
For instance, let the two methods void Person::doit(in int) and void Employee::doit(in int), the method to be invoked in the expression p->doit(x) is decided at evaluation time according to the true type of p:

p := new Person();
p->doit(1); // Person::doit(in int) is invoked

p := new Employee();
p->doit(1); // Employee::doit(in int) is invoked

To invoke a method, the following conditions must be realize:

1. the object or oid on which the method is applied must be an instance of a class, for instance X.
2. the name of the invoked method, the number and the type of arguments must be compatible with an existing method in the class X,
3. the result type must match the expected type in the expression.

For instance, let the methods int compute(in int, int float) and int compute(in int, in float, in int[], out string) in the class X. To invoke the first method on an instance of X, one needs to apply the method compute to an instance of X with one integer and one float, for instance:

x := new X();

x.compute(1, 2.3);
x.compute(a := fact(10), float(fib(10)));

To invoke the second method on an instance of X, one needs to apply the method compute to an instance of X with an integer, a float, an ordered collection of integer and a reference to a variable, for instance:

x.compute(1, 23.4, list(1, 2, 3, 4), &a);

The following table shows the mapping (which defines the compatibility) between the ODL and the OQL types.

ODL/OQL Mapping
ODL Type	OQL Type
in int16	integer
out int16	identifier
inout int16	identifier initialized to an integer
in int32	integer
out int32	identifier
inout int32	identifier initialized to an integer
in int64	integer
out int64	identifier
inout int64	identifier initialized to an integer
in byte	char
out byte	identifier
inout byte	identifier initialized to a char
in char	char
out char	identifier
inout char	identifier initialized to a char
in string	string
out string	identifier
inout string	identifier initialized to a string
in float	float
out float	identifier
inout float	identifier initialized to a float
in oid	oid
out oid	identifier
inout oid	identifier initialized to a oid
in object *	oid of any class
out object *	identifier
inout object *	identifier initialized to an oid of any class
in X * (X denotes a class instance)	oid of class X
out X * (X denotes a class instance)	identifier
inout X * (X denotes a class instance)	identifier initialized to a oid of class X
in X *[] (X denotes a class instance)	ordered collection of oid of class X
out X *[] (X denotes a class instance)	identifier
inout X *[] (X denotes a class instance)	identifier initialized to an ordered collection of oid of class X
in X[] (X denotes any ODL type)	ordered collection of atoms bound to X
out X[] (X denotes any ODL type)	identifier
inout X[] (X denotes any ODL type)	identifier initialized to an ordered collection of atoms bound to X

Note: contrary to the ODMG 3 specifications, one currently needs to use parenthesis to invoke a method even if the method has no arguments.

General Information
Operators	.
	->
Syntaxes	expr . expr (expr_list)
	expr -> expr (expr_list)
Type	n-ary
Operand Types	first operand: oid or object, second operand: identifier, other operands: any type
Result Type	type of the atom returned by the method call
Functions	invokes the method denoted by the second operand applied to the object denoted by the first operand, using the other operands as arguments.
	The first operand must denote an EYEDB instance (object or literal) of an agregat including the method whose name is the second operand. The number of arguments and the type of arguments must match one of the methods included in the class of the object denoted by the first operand.
Note	these two operators are identical

Expression Examples
expression	result	comments
p->getOid()	the value of the oid of object denoted by p	as getOid() is a native method of the class object, each object can call this method
img->compute(1, 2.3)	the value returned by the method call	the first operand must denote an EYEDB instance (object or literal) of an agregat including the method whose name is compute
first(select Person.name = "wayne").getSpouse()	the value returned by the method call

Class Method Invocation

OQL allows one to call a class method with or without parameters. The method can be written in C++ or in OQL. As in C++, method calls use a combination of the scope operator and the function call operator. To invoke a class method, the following conditions must be realize:

1. the name of the invoked method, the number and the type of arguments must be compatible with an existing method in the class X,
2. the result type must match the expected type in the expression.

The overloading and the late binding mechanisms are the same as for the instance method invocations.

General Information
Operator	::
Syntaxe	identifier::identifier(expr_list)
Type	n-ary
Operand Types	first operand: identifier, second operand: identifier, other operands: any type
Result Type	type of the atom returned by the method call
Functions	invokes the class method denoted by the second operand applied to the class denoted by the first operand, using the other operands as arguments.
	The first operand must denote an EYEDB class of an agregat including the class method whose name is the second operand. The number of arguments and the type of arguments must match one of the class methods included in the class denoted by the first operand.

Expression Examples
expression	result	comments
EyeDB::getVersion()	2.8.7	getVersion() is a native static method of the class EyeDB
Person::checkName("johnny")	the value returned by the method call	the class method checkName must exist in the class Person and must take one and only one input string argument.