Thread: Simple factory pattern

  1. #1
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    Simple factory pattern

    Hello,

    how can pass a value to the constructor of the derived structs in this simple factory pattern? thx

    Code:
    #include <iostream>
    #include <memory>
    #include <vector>
    
    struct B {
    	virtual ~B() = default;
    };
    
    struct D1 : B {
    	int Val;
    	D1(int val) : Val(val) {}
    };
    
    struct D2 : B {
    	int Val;
    	D2(int val) : Val(val) {}
    };
    
    template<typename T> std::unique_ptr<B> factory(int val) {
    	return std::make_unique<T>(val);
    }
    
    int main() {
    	std::vector<std::shared_ptr<B>> vec{ factory<D1>(1), factory<D2>(2) };
    	for(const auto& v : vec ) std::cout << v->Val << '\n';
    }

  2. #2
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    Code:
    #include <iostream>
    #include <memory>
    #include <vector>
     
    struct B {
        int Val;
        B(int val = 0) : Val(val) { }
        virtual ~B() = default;
    };
     
    struct D1 : B {
        D1(int val) : B(val) { }
    };
     
    struct D2 : B {
        D2(int val) : B(val) { }
    };
     
    template<typename T>
    std::unique_ptr<B> makeB(int val) {
        return std::make_unique<T>(T(val));
    }
     
    int main() {
        for (const auto& b : { makeB<D1>(1), makeB<D2>(2) })
            std::cout << b->Val << '\n';
    }
    The world hangs on a thin thread, and that is the psyche of man. - Carl Jung

  3. #3
    C++ Witch laserlight's Avatar
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    Quote Originally Posted by serge
    how can pass a value to the constructor of the derived structs in this simple factory pattern? thx
    You did so successfully.

    What you didn't do successfully was trying to access a member specific to derived classes on the elements of a vector of base class smart pointers:
    Code:
    for(const auto& v : vec ) std::cout << v->Val << '\n';
    Since vec is a vector of B shared pointers, v is a const reference to a B shared pointer, so you cannot write v->Val because B doesn't have a Val member, not even a private one.

    This should have been more obvious to you if you paid attention to error messages: next time, force yourself to read and post the error message, not just post a question and code without any indication as to why you think it doesn't work.

    If you look at john.c's post #2, you can see that one simple solution to this is to move the declaration of Val to the base class, if that fits with your overall design.

    Quote Originally Posted by john.c
    Code:
    return std::make_unique<T>(T(val));
    That explicit construction of a T temporary is not necessary: make_unique has perfect forwarding, so serge's version of the make_unique call is better.
    Last edited by laserlight; 4 Weeks Ago at 02:54 PM.
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  4. #4
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    Thanks to both of you. I realized that I don't need y base class at all, so the problem became simpler. Yes, checking error messages does help in most cases.

  5. #5
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    I have a follow up issue that I cannot solve even after checking a few examples on the net. How do I declare the two entities in a header and a cpp file so that function can reach the destructor? This is to avoid the std::unique_ptr with an incomplete type won't compile error. The solution apparently involves moving the constructors and destructors to the cpp file. But this I do not understand, since both are contained in this file already.

    Code:
    struct Engine
    {
    	const int Id;
    	const int Mounts;
    	constexpr Engine(int id, int mounts) : Id(id), Mounts(mounts) {}
    	~Engine() = default;
    	Engine(const Engine&) = delete;
    	Engine(Engine&&) = delete;
    };
    
    auto ForgeEngine(int id, int mounts)
    {
    	return std::make_unique<Engine>(id, mounts);
    }

  6. #6
    C++ Witch laserlight's Avatar
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    Quote Originally Posted by serge
    This is to avoid the std::unique_ptr with an incomplete type won't compile error.
    Post the corresponding error message, don't just refer to it.

    This compiles for me without even a warning on g++ 7.4.0:
    Code:
    #include <iostream>
    #include <memory>
     
    struct Engine
    {
        const int Id;
        const int Mounts;
        constexpr Engine(int id, int mounts) : Id(id), Mounts(mounts) {}
        ~Engine() = default;
        Engine(const Engine&) = delete;
        Engine(Engine&&) = delete;
    };
     
    auto ForgeEngine(int id, int mounts)
    {
        return std::make_unique<Engine>(id, mounts);
    }
    
    int main()
    {
        auto engine = ForgeEngine(1, 2);
        std::cout << engine->Id << ", " << engine->Mounts << std::endl;
    }
    I note that if you want to disable the copy and move constructors, then you probably should disable the copy and move assignment operators too.
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  7. #7
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    Quote Originally Posted by laserlight View Post
    I note that if you want to disable the copy and move constructors, then you probably should disable the copy and move assignment operators too.
    You mean like this?

    Code:
    Engine& operator=(const Engine&) = delete;
    Engine& operator=(Engine&&) = delete;
    The problem I had is if I put this in engine.h

    Code:
    #pragma once
    #include<memory>
    
    struct Engine;
    
    std::unique_ptr<Engine> ForgeEngine(int id, int mounts);
    and this in engine.cpp

    Code:
    #include "engine.h"
    
    struct Engine
    {
    	const int Id;
    	const int Mounts;
    	constexpr Engine(int id, int mounts) : Id(id), Mounts(mounts) {}
    	~Engine() = default;
    	Engine(const Engine&) = delete;
    	Engine(Engine&&) = delete;
    };
    
    std::unique_ptr<Engine> ForgeEngine(int id, int mounts)
    {
    	return std::make_unique<Engine>(id, mounts);
    }
    I get these errors:

    Error C2338 can't delete an incomplete type

    Warning C4150 deletion of pointer to incomplete type 'Engine'; no destructor called

    follows by
    Error C2027 use of undefined type 'Engine'

  8. #8
    C++ Witch laserlight's Avatar
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    Then don't do that

    The class definition should go in the header unless you're trying for the pimpl (opaque pointer) idiom... but in that case in C++ you would still have a class wrapper; it's just the implementation that's hidden in the source file.

    You could even put ForgeEngine's implementation in the header by declaring it as inline.
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

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