CODEWITHSUNDEEP
c++exceptiondesign-patternssmart-pointers
Thorvas
Thorvas

Reputation: 73

Exception thrown while trying to use smart pointers

A moments ago, I was exercising with design patterns, and recently, I've tried to implement a Factory Method pattern. My friend told me that I should always use smart pointers, so I've tried it, but my compiler throws an exception called "Access Violation". What am I doing wrong? There is one interface called "Shape", three inheriting classes from it, Client class and main function. I've tried to do everything with smart pointers, but I'm a little bit unsure, if I'm doing it right.

#include <iostream>
#include <memory>

class Shape
{
public:
    virtual void printShape() = 0;
    static std::shared_ptr<Shape> Create(int num);
};

class Triangle : public Shape
{
public:
    virtual void printShape()
    {
        std::cout << "This is Triangle. \n";
    }
};

class Circle : public Shape
{
public:
    virtual void printShape()
    {
        std::cout << "This is Circle. \n";
    }
};

class Rectangle : public Shape
{
public:
    virtual void printShape()
    {
        std::cout << "This is Rectangle. \n";
    }
};

class Client
{
private:
    std::shared_ptr<Shape> shape;
public:
    Client()
    {
        shape=Shape::Create(1);
    }
    std::shared_ptr<Shape>getShape()
    {
        return shape;
    }
};

std::shared_ptr<Shape> Shape::Create(int num)
{
    switch (num)
    {
    case 1:
        return std::shared_ptr<Circle>();
        break;
    case 2:
        return std::shared_ptr<Triangle>();
        break;
    case 3:
        return std::shared_ptr<Rectangle>();
        break;
    }
}

int main()
{
    std::shared_ptr<Client> client;
    std::shared_ptr<Shape> shape = client->getShape();
    shape->printShape();
}

Upvotes: 2

Views: 100

Answers (2)

songyuanyao
songyuanyao

Reputation: 172864

The return statements like return std::shared_ptr<Circle>(); just returns an empty std::shared_ptr, which contains nothing. Dereference on it like shape->printShape(); leads to UB.

You should construct an object and make it managed by std::shared_ptr. E.g.

std::shared_ptr<Shape> Shape::Create(int num)
{
    switch (num)
    {
    case 1:
        return std::make_shared<Circle>();
        break;
    case 2:
        return std::make_shared<Triangle>();
        break;
    case 3:
        return std::make_shared<Rectangle>();
        break;
    }
}

And also for client.

std::shared_ptr<Client> client = std::make_shared<Client>();

Or just

Client client; // it seems no need to use shared_pointer for client
std::shared_ptr<Shape> shape = client.getShape();

Upvotes: 6

libhet
libhet

Reputation: 21

You make pointers without objects You have to write

return std::shared_ptr<Circle>(new Circle());

Or better use std::make_shared

return std::make_shared<Circle>();

Upvotes: 2

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