Uniform access principle

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The Uniform Access Principle was put forth by Bertrand Meyer. It states "All services offered by a module should be available through a uniform notation, which does not betray whether they are implemented through storage or through computation." This principle applies generally to object-oriented programming languages. In simpler form, it states that there should be no difference between working with an attribute, precomputed property, or method/query.

While most examples focus on the "read" aspect of the principle, Meyer shows that the "write" implications of the principle are harder to deal with in his monthly column on the Eiffel programming language official website.

Many languages have various degrees of support for UAP, where some of the implementations violate the spirit of UAP. The inclusion of 'properties' in some programming languages is another way to address the problem that Meyer discusses. Properties don't provide a uniform notation, but they do make the call to the method which provides a service opaque.

[edit] UAP Example

If a language allows access to a variable via dot-notation and assignment

Foo.bar = 5 //Assigns 5 to the object variable "bar"

then these operations should be the same :

//Assume print displays the variable passed to it, with or without parens
//Assume Foo.bar = 5 for now
print Foo.bar
print Foo.bar()

When executed, should display :

5
5

This allows the object to still hide information as well as being easy to access.

The same should be true for setting the data.

Foo.bar = 5
Foo.bar(5)
//These should achieve the same goal

[edit] Language Examples

[edit] Ruby

class Foo
  attr_reader :x
  def initialize(x)
    @x = x
  end
  def squared_x
    return @x * @x
  end
end
 
y = Foo.new(2)
puts y.x
puts y.squared_x

This outputs:

2
4

Note how even though x is an attribute and squared_x is a parameterless method call, they are accessed the same way.

Ruby enforces UAP, the attr_reader :x call only acts as syntactic sugar for generating an accessor method x. There is no way in Ruby to retrieve an instance variable from an object without calling a method on it.

[edit] Python

The following example uses python properties.

class Foo(object):
    def __init__(self, x):
        self.setx(x)
 
    def getx(self):
        return self.__x
 
    def setx(self, x):
        if not isinstance(x, int):
            raise ValueError('Not an integer')
        self.__x = x
 
    def getsquared_x(self):
        return self.x * self.x
 
    x = property(getx,setx, doc="x attribute of Foo object")
    squared_x = property(getsquared_x, doc="getter for squared x")
 
y = Foo(2)
print y.x
print y.squared_x

This outputs:

2
4

Python properties may be used to allow a method to be invoked with the same syntax as accessing an attribute. Whereas Meyer's UAP would have a single notation for both attribute access and method invocation (method invocation syntax), a language with support for properties still supports separate notations for attribute and method access. Properties allow the attribute notation to be used to invoke a method where that is desirable.

[edit] PHP

class Foo{
    private $x;
 
    function __construct($x){
        $this->x = $x;
    }
 
    function x(){
        return $this->x;
    }
 
    function squared_x(){
        return $this->x * $this->x;
    }
}
 
$y = new Foo(2);
echo $y->x();
echo $y->squared_x();

This outputs:

2
4

Observation: There are many other ways to achieve the same functionality in PHP, for example making x public and accessing it directly or using magic methods function __get($variable)

[edit] See also

• The UniformAccessPrinciple on the c2 wiki
• http://www.eiffel.com/general/monthly_column/2005/Sept_October.html

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