How can I convert std::make_unique<derived>() to std::unique_ptr<base>

Question

I'm trying to build a factory method with self-registration based on ( https://www.codetg.com/article/7r1QnR43bm3ZogBJ.html ), which registers logic operations. But I can't figure out how to convert std::make_unique to std::make_unique. I'm always getting the same error:

return': cannot convert from 'std::unique_ptr<T1,std::default_delete<_Ty>>' to 'std::unique_ptr<LogicOperation,std::default_delete<_Ty>>

I'm still a noob on the subject of unique pointer but I've read on cppreference.com

If T is a derived class of some base B, then std::unique_ptr<T> is implicitly convertible to std::unique_ptr<B>. 
The default deleter of the resulting std::unique_ptr<B> will use operator delete for B, 
leading to undefined behavior unless the destructor of B is virtual.

I have tried instead of making a lambda function,to use std::move() as shown in other examples on stackoverflow. But that does not same to work either.

main

int main()
{
    Signal a;
    Signal b;
    a.setState(1);
    b.setState(0);
    std::unique_ptr<LogicOperation> logic = LogicOperationFactory::Create("AND");
    bool x[2] = { a.getState(), b.getState() };
    bool y = logic->operation(x, 2); // do and operation
}

LogicOperation.h

class LogicOperation
{
public:
    LogicOperation() = default;
    virtual ~LogicOperation() = default;
public:
    virtual bool operation(bool*, uint8_t count) = 0;
};

LogicOperationFactory.h:

        using TCreateMethod = std::function<std::unique_ptr<LogicOperation>()>;
    template<class T1>
    static bool Register(const std::string name)
    {
        std::map<std::string, TCreateMethod>::iterator it;
        it = s_methods.find(name);

        if (it != s_methods.end())
            return false;

        s_methods[name] = []() -> std::unique_ptr<LogicOperation> {
            // Constructs an object of type T and wraps it in a std::unique_ptr
            return std::make_unique<T1>(); // use default constructor
        };

        return true;
    }

LogicAndOperation.cpp

class LogicAndOperation :
    public virtual LogicOperation
{
public:
    LogicAndOperation() = default;
    virtual ~LogicAndOperation() = default;

    bool operation(bool* signals, uint8_t count) override;
private:
    static bool s_registered;
};

bool LogicAndOperation::s_registered =
LogicOperationFactory::Register<LogicAndOperation>("AND");

Can someone please explain to me, how I can make std::unique_ptr from a derived class (LogicAndOperation)?

Answer 1

Given the example code, I cannot see the issue.

This compiled and ran in C++14 mode (Clang 10). I filled-in some of the gaps that our example code was lacking. I cannot see your LogicOperationFactory::Create() function; is that where your issue is?

#include <cassert>
#include <cstdint>
#include <functional>
#include <iostream>
#include <map>
#include <memory>

class LogicOperation
{
public:
    virtual ~LogicOperation() = default;
    virtual bool operation(bool*, uint8_t count) = 0;
};

using TCreateMethod = std::function<std::unique_ptr<LogicOperation>()>;

class LogicOperationFactory
{
    static std::map<std::string, TCreateMethod> s_methods;

public:
    template<class T>
    static bool Register(const std::string& name)
    {
        std::map<std::string, TCreateMethod>::iterator it;
        it = s_methods.find(name);

        if (it != s_methods.end())
            return false;

        s_methods[name] = []() -> std::unique_ptr<LogicOperation> {
            // Constructs an object of type T and wraps it in a std::unique_ptr
            return std::make_unique<T>(); // use default constructor
        };

        return true;
    }

    static std::unique_ptr<LogicOperation> Create(const std::string& name)
    {
        auto iter = s_methods.find(name);
        return (iter != s_methods.end()) ? (iter->second)() : nullptr;
    }
};

std::map<std::string, TCreateMethod> LogicOperationFactory::s_methods;

class FooLogic : public LogicOperation
{
public:
    bool operation(bool*, uint8_t) override {
        std::cout << "FooLogic::operation" << std::endl;
        return true;
    }
};

class BarLogic : public LogicOperation
{
public:
    bool operation(bool*, uint8_t) override {
        std::cout << "BarLogic::operation" << std::endl;
        return true;
    }
};

static bool s_registeredFooLogic = LogicOperationFactory::Register<FooLogic>("FooLogic");
static bool s_registeredBarLogic = LogicOperationFactory::Register<BarLogic>("BarLogic");

int main() {
    assert(s_registeredFooLogic && s_registeredBarLogic);
    auto bar_logic = LogicOperationFactory::Create("BarLogic");
    bool flag = false;
    bar_logic->operation(&flag, 1);

    auto null_logic = LogicOperationFactory::Create("ThisDoesNotExist");
    assert(nullptr == null_logic);
    return 0;
}