Intent IntentDefine a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it.
Also Known AsPolicy
MotivationMany algorithms exist for breaking a stream of text into lines. Hard-wiring all such algorithms into the classes that require them isn't desirable for several reasons:
We can avoid these problems by defining classes that encapsulate different linebreaking algorithms. An algorithm that's encapsulated in this way is called a strategy.
Suppose a Composition class is responsible for maintaining and updating the linebreaks of text displayed in a text viewer. Linebreaking strategies aren't implemented by the class Composition. Instead, they are implemented separately by subclasses of the abstract Compositor class. Compositor subclasses implement different strategies:
A Composition maintains a reference to a Compositor object. Whenever a Composition reformats its text, it forwards this responsibility to its Compositor object. The client of Composition specifies which Compositor should be used by installing the Compositor it desires into the Composition.
ApplicabilityUse the Strategy pattern when
Structure
Participants Collaborations ConsequencesThe Strategy pattern has the following benefits and drawbacks:
For example, without strategies, the code for breaking text into lines could look like
void Composition::Repair () {
switch (_breakingStrategy) {
case SimpleStrategy:
ComposeWithSimpleCompositor();
break;
case TeXStrategy:
ComposeWithTeXCompositor();
break;
// ...
}
// merge results with existing composition, if necessary
}
The Strategy pattern eliminates this case statement by delegating the linebreaking task to a Strategy object:
void Composition::Repair () {
_compositor->Compose();
// merge results with existing composition, if necessary
}
Code containing many conditional statements often indicates the need to apply the Strategy pattern.
ImplementationConsider the following implementation issues:
One approach is to have Context pass data in parameters to Strategy operations—in other words, take the data to the strategy. This keeps Strategy and Context decoupled. On the other hand, Context might pass data the Strategy doesn't need.
Another technique has a context pass itself as an argument, and the strategy requests data from the context explicitly. Alternatively, the strategy can store a reference to its context, eliminating the need to pass anything at all. Either way, the strategy can request exactly what it needs. But now Context must define a more elaborate interface to its data, which couples Strategy and Context more closely.
The needs of the particular algorithm and its data requirements will determine the best technique.
Context) is
defined as a template class that has a Strategy class as a
parameter:
template
class Context {
void Operation() { theStrategy.DoAlgorithm(); }
// ...
private:
AStrategy theStrategy;
};
The class is then configured with a Strategy class when it's
instantiated:
class MyStrategy {
public:
void DoAlgorithm();
};
Context aContext;
With templates, there's no need to define an abstract class that defines
the interface to the Strategy. Using Strategy as a
template parameter also lets you bind a Strategy to its
Context statically, which can increase efficiency.
Sample CodeWe'll give the high-level code for the Motivation example, which is based on the implementation of Composition and Compositor classes in InterViews [LCI+92].
The Composition class maintains a collection of
Component instances, which represent text and graphical
elements in a document. A composition arranges component objects into
lines using an instance of a Compositor subclass, which
encapsulates a linebreaking strategy. Each component has an
associated natural size, stretchability, and shrinkability. The
stretchability defines how much the component can grow beyond its
natural size; shrinkability is how much it can shrink. The
composition passes these values to a compositor, which uses them to
determine the best location for linebreaks.
class Composition {
public:
Composition(Compositor*);
void Repair();
private:
Compositor* _compositor;
Component* _components; // the list of components
int _componentCount; // the number of components
int _lineWidth; // the Composition's line width
int* _lineBreaks; // the position of linebreaks
// in components
int _lineCount; // the number of lines
};
When a new layout is required, the composition asks its compositor to determine where to place linebreaks. The composition passes the compositor three arrays that define natural sizes, stretchabilities, and shrinkabilities of the components. It also passes the number of components, how wide the line is, and an array that the compositor fills with the position of each linebreak. The compositor returns the number of calculated breaks.
The Compositor interface lets the composition pass the
compositor all the information it needs. This is an example of
"taking the data to the strategy":
class Compositor {
public:
virtual int Compose(
Coord natural[], Coord stretch[], Coord shrink[],
int componentCount, int lineWidth, int breaks[]
) = 0;
protected:
Compositor();
};
Note that Compositor is an abstract class. Concrete
subclasses define specific linebreaking strategies.
The composition calls its compositor in its Repair
operation. Repair first initializes arrays with the natural
size, stretchability, and shrinkability of each component (the details
of which we omit for brevity). Then it calls on the compositor to
obtain the linebreaks and finally lays out the components according to
the breaks (also omitted):
void Composition::Repair () {
Coord* natural;
Coord* stretchability;
Coord* shrinkability;
int componentCount;
int* breaks;
// prepare the arrays with the desired component sizes
// ...
// determine where the breaks are:
int breakCount;
breakCount = _compositor->Compose(
natural, stretchability, shrinkability,
componentCount, _lineWidth, breaks
);
// lay out components according to breaks
// ...
}
Now let's look at the Compositor subclasses.
SimpleCompositor examines components a line at a time to
determine where breaks should go:
class SimpleCompositor : public Compositor {
public:
SimpleCompositor();
virtual int Compose(
Coord natural[], Coord stretch[], Coord shrink[],
int componentCount, int lineWidth, int breaks[]
);
// ...
};
TeXCompositor uses a more global strategy. It examines a
paragraph at a time, taking into account the components' size
and stretchability. It also tries to give an even "color" to the
paragraph by minimizing the whitespace between components.
class TeXCompositor : public Compositor {
public:
TeXCompositor();
virtual int Compose(
Coord natural[], Coord stretch[], Coord shrink[],
int componentCount, int lineWidth, int breaks[]
);
// ...
};
ArrayCompositor breaks the components into lines at regular
intervals.
class ArrayCompositor : public Compositor {
public:
ArrayCompositor(int interval);
virtual int Compose(
Coord natural[], Coord stretch[], Coord shrink[],
int componentCount, int lineWidth, int breaks[]
);
// ...
};
These classes don't use all the information passed in
Compose. SimpleCompositor ignores the stretchability
of the components, taking only their natural widths into account.
TeXCompositor uses all the information passed to it, whereas
ArrayCompositor ignores everything.
To instantiate Composition, you pass it the compositor
you want to use:
Composition* quick = new Composition(new SimpleCompositor);
Composition* slick = new Composition(new TeXCompositor);
Composition* iconic = new Composition(new ArrayCompositor(100));
Compositor's interface is carefully designed to support all
layout algorithms that subclasses might implement. You don't want to
have to change this interface with every new subclass, because that will
require changing existing subclasses. In general, the Strategy and
Context interfaces determine how well the pattern achieves its intent.
Known UsesBoth ET++ [WGM88] and InterViews use strategies to encapsulate different linebreaking algorithms as we've described.
In the RTL System for compiler code optimization [JML92], strategies define different register allocation schemes (RegisterAllocator) and instruction set scheduling policies (RISCscheduler, CISCscheduler). This provides flexibility in targeting the optimizer for different machine architectures.
The ET++SwapsManager calculation engine framework computes prices for different financial instruments [EG92]. Its key abstractions are Instrument and YieldCurve. Different instruments are implemented as subclasses of Instrument. YieldCurve calculates discount factors, which determine the present value of future cash flows. Both of these classes delegate some behavior to Strategy objects. The framework provides a family of ConcreteStrategy classes for generating cash flows, valuing swaps, and calculating discount factors. You can create new calculation engines by configuring Instrument and YieldCurve with the different ConcreteStrategy objects. This approach supports mixing and matching existing Strategy implementations as well as defining new ones.
The Booch components [BV90] use strategies as template
arguments. The Booch collection classes support three different kinds of
memory allocation strategies: managed (allocation out of a pool),
controlled (allocations/deallocations are protected by locks), and
unmanaged (the normal memory allocator). These strategies are passed as
template arguments to a collection class when it's instantiated. For
example, an UnboundedCollection that uses the unmanaged strategy is
instantiated as UnboundedCollection.
RApp is a system for integrated circuit layout . RApp must lay out and route wires that connect subsystems on the circuit. Routing algorithms in RApp are defined as subclasses of an abstract Router class. Router is a Strategy class.
Borland's ObjectWindows [Bor94] uses strategies in dialogs boxes to ensure that the user enters valid data. For example, numbers might have to be in a certain range, and a numeric entry field should accept only digits. Validating that a string is correct can require a table look-up.
ObjectWindows uses Validator objects to encapsulate validation strategies. Validators are examples of Strategy objects. Data entry fields delegate the validation strategy to an optional Validator object. The client attaches a validator to a field if validation is required (an example of an optional strategy). When the dialog is closed, the entry fields ask their validators to validate the data. The class library provides validators for common cases, such as a RangeValidator for numbers. New client-specific validation strategies can be defined easily by subclassing the Validator class.
Related PatternsFlyweight (195): Strategy objects often make good flyweights.
