9.1 预想方案

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#include <iostream>
#include <sstream>
#include <string>

struct Shape
{
        virtual std::string str() const = 0;
};

struct Circle : Shape
{
        float radius;

        explicit Circle(const float radius)
                : radius{radius} {}

        void resize(float factor) { radius *= factor; }

        std::string str() const override
        {
                std::ostringstream oss;
                oss << "A circle of radius " << radius;
                return oss.str();
        }
};

int main(void)
{
        return 0;
}

9.2 动态装饰器

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#include <iostream>
#include <sstream>
#include <string>
#include <cstdint>

struct Shape
{
        virtual std::string str() const = 0;
};

struct Circle : Shape
{
        float radius;

        explicit Circle(const float radius)
                : radius{radius} {}

        void resize(float factor) { radius *= factor; }

        std::string str() const override
        {
                std::ostringstream oss;
                oss << "A circle of radius " << radius << std::endl;
                return oss.str();
        }
};

struct Square : Shape
{
        float edge;

        explicit Square(const float edge)
                : edge{edge} {}

        void resize(float factor) { edge *= factor; }

        std::string str() const override
        {
                std::ostringstream oss;
                oss << "A Square of edge " << edge << std::endl;
                return oss.str();
        }
};

struct ColoredShape : Shape
{
        Shape& shape;
        std::string color;

        ColoredShape(Shape& shape, const std::string& color)
                : shape{shape}, color{color} {}

        std::string str() const override
        {
                std::ostringstream oss;
                oss << shape.str() << " has the color " << color << std::endl;
                return oss.str();
        }
        void make_dark();
};

void ColoredShape::make_dark() {
        if (constexpr auto dark = "dark"; !color.starts_with(dark))
        {
                color.insert(0, dark);
        }
}

struct TransparentShape : Shape
{
        Shape& shape;
        uint8_t transparentcy;

        TransparentShape(Shape& shape, const uint8_t transparentcy)
         : shape{shape}, transparentcy{transparentcy} {}

        std::string str() const override
        {
                std::ostringstream oss;
                oss << shape.str() << " has "
                  << static_cast<float>(transparentcy) / 255.f * 100.f
                  << "% transparentcy" << std::endl;

                return oss.str();
        }
};

int main(void)
{
        Circle circle{0.5f};
        ColoredShape redCircle{circle, "red"};
        std::cout << redCircle.str();

        redCircle.make_dark();
        std::cout << redCircle.str();


        Square square{3};
        TransparentShape demiSquare{square, 85};
        std::cout << demiSquare.str();

        Circle c{23};
        ColoredShape cs{c, "green"};
        TransparentShape myCircle{cs, 64};
        std::cout << myCircle.str();

        return 0;
}

9.3 静态装饰器

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#include <iostream>
#include <sstream>
#include <string>
#include <cstdint>
#include <type_traits>

struct Shape
{
        virtual std::string str() const = 0;
};

struct Circle : Shape
{
        float radius;

        explicit Circle(const float radius)
                : radius{radius} {}

        void resize(float factor) { radius *= factor; }

        std::string str() const override
        {
                std::ostringstream oss;
                oss << "A circle of radius " << radius << std::endl;
                return oss.str();
        }
};

struct Square : Shape
{
        float edge;

        explicit Square(const float edge)
                : edge{edge} {}

        explicit Square()
                : edge{0} {}

        void resize(float factor) { edge *= factor; }

        std::string str() const override
        {
                std::ostringstream oss;
                oss << "A Square of edge " << edge << std::endl;
                return oss.str();
        }
};
/*
struct ColoredShape : Shape
{
        Shape& shape;
        std::string color;

        ColoredShape(Shape& shape, const std::string& color)
                : shape{shape}, color{color} {}

        std::string str() const override
        {
                std::ostringstream oss;
                oss << shape.str() << " has the color " << color << std::endl;
                return oss.str();
        }
        void make_dark();
};

void ColoredShape::make_dark() {
        if (constexpr auto dark = "dark"; !color.starts_with(dark))
        {
                color.insert(0, dark);
        }
}
*/

template <typename T>
struct ColoredShape : T
{
        static_assert(std::is_base_of_v<Shape, T>, "Template argument must be a Shape");

        ColoredShape(const std::string& color)
                : color{color} {}

        ColoredShape()
                : ColoredShape{} {};

        std::string color;
        std::string str() const override
        {
                std::ostringstream oss;
                oss << T::str() << " has the color " << color << std::endl;
                return oss.str();
        }
};


/*
struct TransparentShape : Shape
{
        Shape& shape;
        uint8_t transparentcy;

        TransparentShape(Shape& shape, const uint8_t transparentcy)
         : shape{shape}, transparentcy{transparentcy} {}

        std::string str() const override
        {
                std::ostringstream oss;
                oss << shape.str() << " has "
                  << static_cast<float>(transparentcy) / 255.f * 100.f
                  << "% transparentcy" << std::endl;

                return oss.str();
        }
};
*/
template <typename T>
struct TransparentShape : T
{
        static_assert(std::is_base_of_v<Shape, T>, "Template argument must be a Shape");

        uint8_t transparentcy;

        TransparentShape(const uint8_t transparentcy)
         : transparentcy{transparentcy} {}

        TransparentShape()
                : transparentcy{} {};

        std::string str() const override
        {
                std::ostringstream oss;
                oss << T::str() << " has "
                  << static_cast<float>(transparentcy) / 255.f * 100.f
                  << "% transparentcy" << std::endl;

                return oss.str();
        }
};

int main(void)
{
/*
        Circle circle{0.5f};
        ColoredShape redCircle{circle, "red"};
        std::cout << redCircle.str();

        redCircle.make_dark();
        std::cout << redCircle.str();


        Square square{3};
        TransparentShape demiSquare{square, 85};
        std::cout << demiSquare.str();

        Circle c{23};
        ColoredShape cs{c, "green"};
        TransparentShape myCircle{cs, 64};
        std::cout << myCircle.str();
*/

        ColoredShape<TransparentShape<Square>> square{"blue"};
//      ColoredShape<Square> square{"blue"};
        square.edge = 2;
        square.transparentcy = 0.5;
        std::cout << square.str();
        return 0;
}

9.4 函数装饰器

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#include <iostream>
#include <functional>

struct Logger
{
        std::function<void()> func;
        std::string name;

        Logger(const std::function<void()>&func, const std::string& name)
                : func{func}, name{name} {}

        void operator()() const
        {
                std::cout << "Entering " << name << "\n";
                func();
                std::cout << "Exiting " << name << "\n";
        }

};

int main(void)
{
        Logger([]() { std::cout << "Hello\n"; }, "HelloFunction")();

        return 0;
}
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#include <iostream>
#include <functional>
#include "stdarg.h"

struct Logger
{
        std::function<void()> func;
        std::string name;

        Logger(const std::function<void()>&func, const std::string& name)
                : func{func}, name{name} {}

        void operator()() const
        {
                std::cout << "Entering " << name << "\n";
                func();
                std::cout << "Exiting " << name << "\n";
        }

};

template <typename Func>
struct Logger2
{
        Func func;
        std::string name;

        Logger2(const Func& func, const std::string& name)
                : func{func}, name{name} {}

        void operator()() const
        {
                std::cout << "Entering" << name << std::endl;
                func();
                std::cout << "Exiting" << name << std::endl;
        }
};

template <typename Func> auto make_logger2(Func func, const std::string& name)
{
                return Logger2<Func>{ func, name };
}

double add(double a, double b)
{
        std::cout << a << "+" << b << "=" << (a + b) << std::endl;
        return a+b;
}

template <typename R, typename... Args>
struct Logger3
{
        Logger3(std::function<R(Args...)> func, const std::string& name)
                : func{func}, name{name} {}

        R operator() (Args ...args)
        {
                std::cout << "Entering " << name << std::endl;
                R result = func(args...);
                std::cout << "Exiting " << name << std::endl;
                return result;
        }

        std::function<R(Args...)> func;
        std::string name;
};

template <typename R, typename... Args>
auto make_logger3(R (*func)(Args...), const std::string& name)
{
        return Logger3<R, Args...>(std::function<R(Args...)>(func), name);
}

int main(void)
{
        Logger([]() { std::cout << "Hello\n"; }, "HelloFunction")();

        auto call = make_logger2([]() { std::cout << "Hello!" << std::endl; },
        "HelloFunction");
        call();

        auto logged_add = make_logger3(add, std::string("Add"));
        auto result = logged_add(2, 3);

        return 0;
}