| Thinking in C++ Vol 2 - Practical Programming |
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Note that the Template Method is the code that stays the
same, and the functions that you override are the code that changes.
However, this change is fixed at compile time via inheritance. Following the
maxim of prefer composition to inheritance, we can use composition to
approach the problem of separating code that changes from code that stays the
same, and produce the Strategy pattern. This approach has a distinct benefit:
at runtime, you can plug in the code that changes. Strategy also adds a
Context which can be a surrogate class that controls the selection and use of
the particular strategy object just like State!
Strategy means just that: you can solve a problem in a number
of ways. Consider the situation where you ve forgotten someone s name. Here are
the different ways you can cope:
//: C10:Strategy.cpp
// The Strategy design pattern.
#include <iostream>
using namespace std;
class NameStrategy {
public:
virtual void greet() = 0;
};
class SayHi : public NameStrategy {
public:
void greet() {
cout << "Hi! How's it going?"
<< endl;
}
};
class Ignore : public NameStrategy {
public:
void greet() {
cout << "(Pretend I don't see you)"
<< endl;
}
};
class Admission : public NameStrategy {
public:
void greet() {
cout << "I'm sorry. I forgot your
name." << endl;
}
};
// The "Context" controls the strategy:
class Context {
NameStrategy& strategy;
public:
Context(NameStrategy& strat) : strategy(strat) {}
void greet() { strategy.greet(); }
};
int main() {
SayHi sayhi;
Ignore ignore;
Admission admission;
Context c1(sayhi), c2(ignore), c3(admission);
c1.greet();
c2.greet();
c3.greet();
} ///:~
Context::greet( ) would normally be more complex;
it s the analog of the Template Method because it contains the code that
doesn t change. But you can see in main( ) that the choice of
strategy can be made at runtime. If you go one step further you can combine
this with the State pattern and change the Strategy during the lifetime of the Context
object.
| Thinking in C++ Vol 2 - Practical Programming |
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