- extracted implementation details(SRP)
- created opportunity for easy change and easy extension
- reduced duplication(DRY)
- Linux Virtual File System？
Example 1
底下的例子中，SortingContext 為 client 端（在此為 main）的 interface，client 可使用不同的 SortingStrategy ，SortingStrategy 是可抽換的，其子類別為不同的 sorting algorithm，將子類別當作參數傳入 SortingContext 裡的 member function 是安全的，因為子類別 pointer 轉成其 parent 類別的 pointer 是安全的。
#include <vector>
#include <memory>
#include <iostream>
class SortingStrategy
{
public:
SortingStrategy(){}
virtual ~SortingStrategy(){}
virtual void sort(std::vector<int>& A){}
};
class BubbleSortStrategy : public SortingStrategy
{
public:
void sort(std::vector<int>& A) override
{
std::cout << "Sorting using Bubble Sort \n";
}
};
class MergeSortStrategy : public SortingStrategy
{
public:
void sort(std::vector<int>& A) override
{
std::cout << "Sorting using Merge Sort \n";
}
};
class SortingContext {
private:
std::unique_ptr<SortingStrategy> sortingStrategy;
public:
SortingContext(std::unique_ptr<SortingStrategy> strategy):
sortingStrategy(std::move(strategy)) {}
void setSortingStrategy(std::unique_ptr<SortingStrategy> strategy) {
this->sortingStrategy = std::move(strategy);
}
void performSort(std::vector<int>& array) {
sortingStrategy.get()->sort(array);
}
};
int main() {
SortingContext* sortingContext = new SortingContext(std::make_unique<BubbleSortStrategy>());
std::vector<int> array1 = {5, 2, 9, 1, 5};
sortingContext->performSort(array1);
// Change strategy to MergeSortStrategy
sortingContext->setSortingStrategy(std::make_unique<MergeSortStrategy>());
std::vector<int> array2 = {8, 3, 7, 4, 2};
sortingContext->performSort(array2); // Output: Sorting using Merge Sort
}要注意這邊不能使用 this->sortingStrategy.reset(strategy.get()) ，會造成 double free，因為上一個 strategy 因為本身就是 unique pointer，因此已經自動被 free 了。
this->sortingStrategy = std::move(strategy);Example 2
#include <memory>
#include <iostream>
class Shape {
public:
Shape() = default;
virtual ~Shape() = default;
virtual void draw() = 0;
};
class DrawStrategy {
public:
DrawStrategy(){}
virtual ~DrawStrategy(){}
virtual void draw(Shape& shape){};
};
class DrawSquareStrategy : public DrawStrategy
{
public:
void draw(Shape& square) override
{
std::cout << "Drawing an Circle! \n";
}
};
class DrawCircleStrategy : public DrawStrategy
{
public:
void draw(Shape& circle) override
{
std::cout << "Drawing an Square! \n";
}
};
class Circle : public Shape
{
private:
std::unique_ptr<DrawCircleStrategy> drawing;
public:
Circle(std::unique_ptr<DrawCircleStrategy> strategy):
drawing(std::move(strategy)) {}
void draw() override {
drawing->draw(*this);
}
};
class Square : public Shape
{
private:
std::unique_ptr<DrawSquareStrategy> drawing;
public:
Square(std::unique_ptr<DrawSquareStrategy> strategy):
drawing(std::move(strategy)) {}
void draw() override {
drawing->draw(*this);
}
};
int main() {
Square* s1 = new Square(std::make_unique<DrawSquareStrategy>());
Circle* c1 = new Circle(std::make_unique<DrawCircleStrategy>());
s1->draw();
c1->draw();
}