for unit testing, I'm trying to create a Factory which meets the following requirements:
Timer and TimerMock)unique_ptrs to a Base class to these objects (e.g. unique_ptr<TimerInterface>)The purpose is to do dependency injection with this factories to be able to exchange objects that are not part of the test with mock objects.
Here's what I have so far:
#include <memory>
#include <iostream>
//"perfect factory method"
//usage: std::unique_ptr<Type> pt(create<Type>(Type-constructor-arguments));
template <typename Type, typename ... ConstructorArgs>
auto create(ConstructorArgs&& ... args){
return std::make_unique<Type>(std::forward<ConstructorArgs>(args)...);
}
//Abstract Factory Base class
template<typename BaseType, typename ... ConstructorArgs>
class IFactory {
public:
virtual ~IFactory() = default;
virtual std::unique_ptr<BaseType> create(ConstructorArgs&& ... args) const = 0;
};
//Abstract Factory class
template <typename BaseType, typename DerivedType, typename ... ConstructorArgs>
class CFactory : public IFactory<BaseType, ConstructorArgs...>
{
public:
using Base = IFactory<BaseType, ConstructorArgs...>;
std::unique_ptr<BaseType> create(ConstructorArgs&& ... args) const override
{
return ::create<DerivedType>(std::forward<ConstructorArgs>(args)...);
}
};
How the real Factory classes are defined:
class TimerInterface {
public:
TimerInterface() = default;
TimerInterface (const char* name);
virtual void whoami() const = 0;
/*...*/
};
class Timer: public TimerInterface {
public:
Timer() = default;
Timer(const char* name) : TimerInterface (name) {}
void whoami() const override { std::cerr << "I'm real!" << std::endl; }
/*...*/
};
class TimerMock : public TimerInterface {
public:
TimerMock () = default;
TimerMock (const char* name) : TimerInterface (name) {}
void whoami() const override { std::cerr << "I fake it!" << std::endl; }
/*...*/
};
using TimerFactory = CFactory<TimerInterface, Timer, const char*>;
using TimerMockFactory = CFactory<TimerInterface, TimerMock, const char*>;
using TimerFactoryInterface = TimerFactory::Base;
And how they are intended to be used:
class ClassUnderTest {
public:
std::unique_ptr<TimerInterface> timer {};
std::unique_ptr<TimerInterface> timer2 {};
ClassUnderTest(const TimerFactoryInterface& factory)
: timer(factory.create("I got a name!"))
//, timer2(factory.create())
{}
};
class Production
{
public:
ClassUnderTest realUsage;
Production() :
realUsage(TimerFactory())
{}
};
class Test
{
public:
ClassUnderTest tested;
Test() :
tested(TimerMockFactory())
{}
};
int main()
{
Production p;
p.realUsage.timer->whoami();
Test t;
t.tested.timer->whoami();
}
My big problem is requirement (4) ClassUnderTest::timer2 can't be created with the same factory that's used for ClassUnderTest::timer, beause the constructor signatur already needs to be known when defining the CFactory class.
Anyone got an idea?
P.S.: "It can't be done" with an explanation is also an acceptable answer, but not my favourite ;)
@SparkyPotato gave me the idea when stating I'd "have to manually list out the possible constructors and generate overloads for create". I just disliked the manually, so I did it by template meta programming. Thanks for the hint!
#include <memory>
#include <iostream>
#include <tuple>
//"perfect factory method"
//usage: std::unique_ptr<Type> pt(create<Type>(Type-constructor-arguments));
template <typename Type, typename ... ConstructorArgs>
auto create(ConstructorArgs&& ... args){
std::cerr << "calling " << __PRETTY_FUNCTION__ << std::endl;
return std::make_unique<Type>(std::forward<ConstructorArgs>(args)...);
}
//Abstract Factory Variadic class. This is also generic case that ends recursion
template <typename BaseType, typename TupleListOfConstructorArgs>
class IFactory
{
static_assert(std::is_same<TupleListOfConstructorArgs, std::tuple<>>::value, "");
public:
//this method shall never be instatiated, it just exists to satisfy the "using BaseFactory::create" in IFactory
template <typename T> void create(){ static_assert(sizeof(BaseType) + sizeof(T) < 0, ""); }
virtual ~IFactory() = default;
};
//Abstract Factory Variadic class specialization to perform inheritance recursion
template <typename BaseType, typename ... CurrentTupleArgs, typename ... TupleListOfConstructorArgs>
class IFactory<BaseType, std::tuple<std::tuple<CurrentTupleArgs...>, TupleListOfConstructorArgs...>> : public IFactory<BaseType, std::tuple<TupleListOfConstructorArgs...>>
{
public:
using BaseFactory = IFactory<BaseType, std::tuple<TupleListOfConstructorArgs...>>;
using BaseFactory::create;
virtual std::unique_ptr<BaseType> create(const CurrentTupleArgs& ... args) const = 0;
};
//Concrete Factory Variadic class. This is also generic case that ends inheritance recursion
template <typename BaseType, typename DerivedType, typename TupleListOfConstructorArgs, typename FullTupleListOfConstructorArgs>
class CFactory : public IFactory<BaseType, FullTupleListOfConstructorArgs>
{
static_assert(std::is_same<TupleListOfConstructorArgs, std::tuple<>>::value, "");
public:
using Base = IFactory<BaseType, FullTupleListOfConstructorArgs>;
};
//Concrete Factory Variadic class specialization to perform inheritance recursion
template <typename BaseType, typename DerivedType, typename ... CurrentTupleArgs, typename ... TupleListOfConstructorArgs, typename FullTupleListOfConstructorArgs>
class CFactory<BaseType, DerivedType, std::tuple<std::tuple<CurrentTupleArgs...>, TupleListOfConstructorArgs...>, FullTupleListOfConstructorArgs> : public CFactory<BaseType, DerivedType, std::tuple<TupleListOfConstructorArgs...>, FullTupleListOfConstructorArgs>
{
public:
using BaseFactory = CFactory<BaseType, DerivedType, std::tuple<TupleListOfConstructorArgs...>, FullTupleListOfConstructorArgs>;
using BaseFactory::create;
std::unique_ptr<BaseType> create(const CurrentTupleArgs& ... args) const override
{
std::cerr << "calling " << __PRETTY_FUNCTION__ << std::endl;
return ::create<DerivedType>(args...);
}
};
template <typename BaseType, typename DerivedType, typename TupleListOfConstructorArgs>
using CFactoryFrontend = CFactory<BaseType, DerivedType, TupleListOfConstructorArgs, TupleListOfConstructorArgs>;
class TimerInterface {
public:
TimerInterface() = default;
virtual ~TimerInterface() = default;
TimerInterface (const char* name) {}
TimerInterface(int& x, const char* name) { std::cerr << "calling " << __PRETTY_FUNCTION__ << std::endl; }
TimerInterface(const int& x, const char* name) { std::cerr << "calling " << __PRETTY_FUNCTION__ << std::endl; }
virtual void whoami() const = 0;
/*...*/
};
class Timer: public TimerInterface {
public:
Timer() = default;
Timer(const char* name) : TimerInterface (name) {}
Timer(int& x, const char* name) : TimerInterface(x, name) {}
Timer(const int& x, const char* name) : TimerInterface(x, name) {}
void whoami() const override { std::cerr << "I'm real!" << std::endl; }
/*...*/
};
class TimerMock : public TimerInterface {
public:
TimerMock () = default;
TimerMock (const char* name) : TimerInterface (name) {}
TimerMock (int& x, const char* name) : TimerInterface(x, name) {}
TimerMock (const int& x, const char* name) : TimerInterface(x, name) {}
void whoami() const override { std::cerr << "I fake it!" << std::endl; }
/*...*/
};
//using TimerInterfaceConstructors = std::tuple<std::tuple<>, std::tuple<const char*>>;
using Constructors = std::tuple<std::tuple<>, std::tuple<const char*>, std::tuple<int&, const char*>, std::tuple<const int&, const char*>>;
using TestFactory = CFactoryFrontend<TimerInterface, Timer, Constructors>;
using TimerFactory = CFactoryFrontend<TimerInterface, Timer, Constructors>;
using TimerMockFactory = CFactoryFrontend<TimerInterface, TimerMock, Constructors>;
using TimerFactoryInterface = TimerFactory::Base;
class ClassUnderTest {
public:
std::unique_ptr<TimerInterface> timer {};
std::unique_ptr<TimerInterface> timer2 {};
ClassUnderTest(const TimerFactoryInterface& factory)
: timer(factory.create("I got a name!"))
, timer2(factory.create())
{}
};
class Production
{
public:
ClassUnderTest realUsage;
Production() :
realUsage(TimerFactory())
{}
};
class Test
{
public:
ClassUnderTest tested;
Test() :
tested(TimerMockFactory())
{}
};
int main()
{
Production p;
p.realUsage.timer->whoami();
Test t;
t.tested.timer->whoami();
TestFactory tf;
TimerFactoryInterface& tfi(tf);
const char* x = "X";
tfi.create();
tfi.create(x);
int y;
const int cy = 17;
tfi.create(y, x);
tfi.create(cy, "name");
::create<Timer>(x);
}
It compiles using GCC-6 and newer and clang with -std=c++-14