C++ Constructor from Type [duplicate] - c++

Question

I have a file: Base.h
class Base;
class DerivedA : public Base;
class DerivedB : public Base;
/*etc...*/
and another file: BaseFactory.h
#include "Base.h"
class BaseFactory
{
public:
BaseFactory(const string &sClassName){msClassName = sClassName;};
Base * Create()
{
if(msClassName == "DerivedA")
{
return new DerivedA();
}
else if(msClassName == "DerivedB")
{
return new DerivedB();
}
else if(/*etc...*/)
{
/*etc...*/
}
};
private:
string msClassName;
};
/*etc.*/
Is there a way to somehow convert this string to an actual type (class), so that BaseFactory wouldn't have to know all the possible Derived classes, and have if() for each one of them? Can I produce a class from this string?
I think this can be done in C# through Reflection. Is there something similar in C++?

Answer 1

Nope, there is none, unless you do the mapping yourself. C++ has no mechanism to create objects whose types are determined at runtime. You can use a map to do that mapping yourself, though:
template<typename T> Base * createInstance() { return new T; }
typedef std::map<std::string, Base*(*)()> map_type;
map_type map;
map["DerivedA"] = &createInstance<DerivedA>;
map["DerivedB"] = &createInstance<DerivedB>;
And then you can do
return map[some_string]();
Getting a new instance. Another idea is to have the types register themself:
// in base.hpp:
template<typename T> Base * createT() { return new T; }
struct BaseFactory {
typedef std::map<std::string, Base*(*)()> map_type;
static Base * createInstance(std::string const& s) {
map_type::iterator it = getMap()->find(s);
if(it == getMap()->end())
return 0;
return it->second();
}
protected:
static map_type * getMap() {
// never delete'ed. (exist until program termination)
// because we can't guarantee correct destruction order
if(!map) { map = new map_type; }
return map;
}
private:
static map_type * map;
};
template<typename T>
struct DerivedRegister : BaseFactory {
DerivedRegister(std::string const& s) {
getMap()->insert(std::make_pair(s, &createT<T>));
}
};
// in derivedb.hpp
class DerivedB {
...;
private:
static DerivedRegister<DerivedB> reg;
};
// in derivedb.cpp:
DerivedRegister<DerivedB> DerivedB::reg("DerivedB");
You could decide to create a macro for the registration
#define REGISTER_DEC_TYPE(NAME) \
static DerivedRegister<NAME> reg
#define REGISTER_DEF_TYPE(NAME) \
DerivedRegister<NAME> NAME::reg(#NAME)
I'm sure there are better names for those two though. Another thing which probably makes sense to use here is shared_ptr.
If you have a set of unrelated types that have no common base-class, you can give the function pointer a return type of boost::variant<A, B, C, D, ...> instead. Like if you have a class Foo, Bar and Baz, it looks like this:
typedef boost::variant<Foo, Bar, Baz> variant_type;
template<typename T> variant_type createInstance() {
return variant_type(T());
}
typedef std::map<std::string, variant_type (*)()> map_type;
A boost::variant is like an union. It knows which type is stored in it by looking what object was used for initializing or assigning to it. Have a look at its documentation here. Finally, the use of a raw function pointer is also a bit oldish. Modern C++ code should be decoupled from specific functions / types. You may want to look into Boost.Function to look for a better way. It would look like this then (the map):
typedef std::map<std::string, boost::function<variant_type()> > map_type;
std::function will be available in the next version of C++ too, including std::shared_ptr.

Answer 2

No there isn't. My preferred solution to this problem is to create a dictionary which maps name to creation method. Classes that want to be created like this then register a creation method with the dictionary. This is discussed in some detail in the GoF patterns book.

Answer 3

The short answer is you can't. See these SO questions for why:
Why does C++ not have reflection?
How can I add reflection to a C++ application?

Answer 4

I have answered in another SO question about C++ factories. Please see there if a flexible factory is of interest. I try to describe an old way from ET++ to use macros which has worked great for me.
ET++ was a project to port old MacApp to C++ and X11. In the effort of it Eric Gamma etc started to think about Design Patterns

Answer 5

boost::functional has a factory template which is quite flexible: http://www.boost.org/doc/libs/1_54_0/libs/functional/factory/doc/html/index.html
My preference though is to generate wrapper classes which hide the mapping and object creation mechanism. The common scenario I encounter is the need to map different derived classes of some base class to keys, where the derived classes all have a common constructor signature available. Here is the solution I've come up with so far.
#ifndef GENERIC_FACTORY_HPP_INCLUDED
//BOOST_PP_IS_ITERATING is defined when we are iterating over this header file.
#ifndef BOOST_PP_IS_ITERATING
//Included headers.
#include <unordered_map>
#include <functional>
#include <boost/preprocessor/iteration/iterate.hpp>
#include <boost/preprocessor/repetition.hpp>
//The GENERIC_FACTORY_MAX_ARITY directive controls the number of factory classes which will be generated.
#ifndef GENERIC_FACTORY_MAX_ARITY
#define GENERIC_FACTORY_MAX_ARITY 10
#endif
//This macro magic generates GENERIC_FACTORY_MAX_ARITY + 1 versions of the GenericFactory class.
//Each class generated will have a suffix of the number of parameters taken by the derived type constructors.
#define BOOST_PP_FILENAME_1 "GenericFactory.hpp"
#define BOOST_PP_ITERATION_LIMITS (0,GENERIC_FACTORY_MAX_ARITY)
#include BOOST_PP_ITERATE()
#define GENERIC_FACTORY_HPP_INCLUDED
#else
#define N BOOST_PP_ITERATION() //This is the Nth iteration of the header file.
#define GENERIC_FACTORY_APPEND_PLACEHOLDER(z, current, last) BOOST_PP_COMMA() BOOST_PP_CAT(std::placeholders::_, BOOST_PP_ADD(current, 1))
//This is the class which we are generating multiple times
template <class KeyType, class BasePointerType BOOST_PP_ENUM_TRAILING_PARAMS(N, typename T)>
class BOOST_PP_CAT(GenericFactory_, N)
{
public:
typedef BasePointerType result_type;
public:
virtual ~BOOST_PP_CAT(GenericFactory_, N)() {}
//Registers a derived type against a particular key.
template <class DerivedType>
void Register(const KeyType& key)
{
m_creatorMap[key] = std::bind(&BOOST_PP_CAT(GenericFactory_, N)::CreateImpl<DerivedType>, this BOOST_PP_REPEAT(N, GENERIC_FACTORY_APPEND_PLACEHOLDER, N));
}
//Deregisters an existing registration.
bool Deregister(const KeyType& key)
{
return (m_creatorMap.erase(key) == 1);
}
//Returns true if the key is registered in this factory, false otherwise.
bool IsCreatable(const KeyType& key) const
{
return (m_creatorMap.count(key) != 0);
}
//Creates the derived type associated with key. Throws std::out_of_range if key not found.
BasePointerType Create(const KeyType& key BOOST_PP_ENUM_TRAILING_BINARY_PARAMS(N,const T,& a)) const
{
return m_creatorMap.at(key)(BOOST_PP_ENUM_PARAMS(N,a));
}
private:
//This method performs the creation of the derived type object on the heap.
template <class DerivedType>
BasePointerType CreateImpl(BOOST_PP_ENUM_BINARY_PARAMS(N,const T,& a))
{
BasePointerType pNewObject(new DerivedType(BOOST_PP_ENUM_PARAMS(N,a)));
return pNewObject;
}
private:
typedef std::function<BasePointerType (BOOST_PP_ENUM_BINARY_PARAMS(N,const T,& BOOST_PP_INTERCEPT))> CreatorFuncType;
typedef std::unordered_map<KeyType, CreatorFuncType> CreatorMapType;
CreatorMapType m_creatorMap;
};
#undef N
#undef GENERIC_FACTORY_APPEND_PLACEHOLDER
#endif // defined(BOOST_PP_IS_ITERATING)
#endif // include guard
I am generally opposed to heavy macro use, but I've made an exception here. The above code generates GENERIC_FACTORY_MAX_ARITY + 1 versions of a class named GenericFactory_N, for each N between 0 and GENERIC_FACTORY_MAX_ARITY inclusive.
Using the generated class templates is easy. Suppose you want a factory to create BaseClass derived objects using a string mapping. Each of the derived objects take 3 integers as constructor parameters.
#include "GenericFactory.hpp"
typedef GenericFactory_3<std::string, std::shared_ptr<BaseClass>, int, int int> factory_type;
factory_type factory;
factory.Register<DerivedClass1>("DerivedType1");
factory.Register<DerivedClass2>("DerivedType2");
factory.Register<DerivedClass3>("DerivedType3");
factory_type::result_type someNewObject1 = factory.Create("DerivedType2", 1, 2, 3);
factory_type::result_type someNewObject2 = factory.Create("DerivedType1", 4, 5, 6);
The GenericFactory_N class destructor is virtual to allow the following.
class SomeBaseFactory : public GenericFactory_2<int, BaseType*, std::string, bool>
{
public:
SomeBaseFactory() : GenericFactory_2()
{
Register<SomeDerived1>(1);
Register<SomeDerived2>(2);
}
};
SomeBaseFactory factory;
SomeBaseFactory::result_type someObject = factory.Create(1, "Hi", true);
delete someObject;
Note that this line of the generic factory generator macro
#define BOOST_PP_FILENAME_1 "GenericFactory.hpp"
Assumes the generic factory header file is named GenericFactory.hpp

Answer 6

Detail solution for registering the objects, and accessing them with string names.
common.h:
#ifndef COMMON_H_
#define COMMON_H_
#include<iostream>
#include<string>
#include<iomanip>
#include<map>
using namespace std;
class Base{
public:
Base(){cout <<"Base constructor\n";}
virtual ~Base(){cout <<"Base destructor\n";}
};
#endif /* COMMON_H_ */
test1.h:
/*
* test1.h
*
* Created on: 28-Dec-2015
* Author: ravi.prasad
*/
#ifndef TEST1_H_
#define TEST1_H_
#include "common.h"
class test1: public Base{
int m_a;
int m_b;
public:
test1(int a=0, int b=0):m_a(a),m_b(b)
{
cout <<"test1 constructor m_a="<<m_a<<"m_b="<<m_b<<endl;
}
virtual ~test1(){cout <<"test1 destructor\n";}
};
#endif /* TEST1_H_ */
3. test2.h
#ifndef TEST2_H_
#define TEST2_H_
#include "common.h"
class test2: public Base{
int m_a;
int m_b;
public:
test2(int a=0, int b=0):m_a(a),m_b(b)
{
cout <<"test1 constructor m_a="<<m_a<<"m_b="<<m_b<<endl;
}
virtual ~test2(){cout <<"test2 destructor\n";}
};
#endif /* TEST2_H_ */
main.cpp:
#include "test1.h"
#include "test2.h"
template<typename T> Base * createInstance(int a, int b) { return new T(a,b); }
typedef std::map<std::string, Base* (*)(int,int)> map_type;
map_type mymap;
int main()
{
mymap["test1"] = &createInstance<test1>;
mymap["test2"] = &createInstance<test2>;
/*for (map_type::iterator it=mymap.begin(); it!=mymap.end(); ++it)
std::cout << it->first << " => " << it->second(10,20) << '\n';*/
Base *b = mymap["test1"](10,20);
Base *b2 = mymap["test2"](30,40);
return 0;
}
Compile and Run it (Have done this with Eclipse)
Output:
Base constructor
test1 constructor m_a=10m_b=20
Base constructor
test1 constructor m_a=30m_b=40

Answer 7

Tor Brede Vekterli provides a boost extension that gives exactly the functionality you seek. Currently, it is slightly awkward fitting with current boost libs, but I was able to get it working with 1.48_0 after changing its base namespace.
http://arcticinteractive.com/static/boost/libs/factory/doc/html/factory/factory.html#factory.factory.reference
In answer to those who question why such a thing (as reflection) would be useful for c++ - I use it for interactions between the UI and an engine - the user selects an option in the UI, and the engine takes the UI selection string, and produces an object of the desired type.
The chief benefit of using the framework here (over maintaining a fruit-list somewhere) is that the registering function is in each class's definition (and only requires one line of code calling the registration function per registered class) - as opposed to a file containing the fruit-list, which must be manually added to each time a new class is derived.
I made the factory a static member of my base class.

Answer 8

Meaning reflection as in Java.
there is some info here:
http://msdn.microsoft.com/en-us/library/y0114hz2(VS.80).aspx
Generally speaking, search google for "c++ reflection"

Answer 9

This is the factory pattern. See wikipedia (and this example). You cannot create a type per se from a string without some egregious hack. Why do you need this?

Answer 10

Yes, it is possible, without the use of frameworks and macros, just getting the memory address of the class methods and constructors. You can retrieve them from the map generated by the linker, when configured for this action.
visit this site
https://ealaframework.no-ip.org/wiki/page/c.reference

Answer 11

A C++11-style full example:
// Base.h
class Base;
class DerivedA : public Base;
class DerivedB : public Base;
// BaseFactory.h
class BaseFactory
{
public:
static BaseFactory& get() {
static BaseFactory singleton;
return singleton;
}
virtual ~BaseFactory() {};
BaseFactory(const BaseFactory&) = delete;
BaseFactory(BaseFactory&&) = delete;
template <class DerivedClass>
static std::shared_ptr<Base> creator()
{
return std::shared_ptr<Base>(new DerivedClass());
}
template <class DerivedClass>
void register_class(const std::string& class_name)
{
if (name_to_creator_map.find(class_name) == name_to_creator_map.end())
{
std::function<std::shared_ptr<Base>(void)> functor = &BaseFactory::template creator<DerivedClass>;
name_to_creator_map.emplace(class_name, functor);
}
}
std::shared_ptr<Base> create(const std::string& class_name) const;
private:
BaseFactory();
std::map<std::string, std::function<std::shared_ptr<Base>(void)>> name_to_creator_map;
};
// example.cpp using BaseFactory
BaseFactory::get().register_class<DerivedA>("DerivedA");
BaseFactory::get().register_class<DerivedB>("DerivedB");
auto a_obj = BaseFactory::get().create("DerivedA");
auto b_obj = BaseFactory::get().create("DerivedB");

Answer 12

There is a ready-to-use reflection library https://www.rttr.org/ . You can easily instantiate class by string with it.
struct MyStruct { MyStruct() {}; void func(double) {}; int data; };
RTTR_REGISTRATION
{
registration::class_<MyStruct>("MyStruct")
.constructor<>()
.property("data", &MyStruct::data)
.method("func", &MyStruct::func);
}
type t = type::get_by_name("MyStruct");
variant var = t.create();