Use String-To-Class Lookup table in C++ to instantiate classes

Question

Looking for a way to avoid a massive IF/ELSE and use a lookup table to resolve strings to particular classes to instantiate, that all derive from a base class. Is something like this possible, and if so, how?

typedef struct BaseClass
{
} BaseClass;

typedef struct DerivedClassOne : BaseClass
{
} DerivedClassOne;

typedef struct DerivedClassTwo : BaseClass
{
} DerivedClassTwo;

typedef struct
{
    const char *name;
    BaseClass class;
} LookupList;

LookupList list[] = {
    {"ClassOne", DerivedClassOne},
    {"ClassTwo", DerivedClassTwo}
};

BaseClass *InstantiateFromString(char *name)
{
    int i;
    for (i = 0; i < 2; i++)
    {
        if (!strcmp(name, list[i].name))
            return new list[i].class();
    }
}

int main (int argc, char *argv[])
{
    BaseClass *myObjectFromLookup = InstantiateFromString("ClassOne");
}

Answer 1

If your compiler is compatible with C++11, you can easily do that with lambdas and std::map:

#include <iostream>
#include <string>
#include <map>
#include <functional>

using namespace std;

struct BaseClass {virtual void foo()=0;};
struct DerivedClass1 : public BaseClass {void foo() {cout << "1" << endl;}};
struct DerivedClass2 : public BaseClass {void foo() {cout << "2" << endl;}};

// Here is the core of the solution: this map of lambdas does all the "magic"
map<string,function<BaseClass*()> > factory {
    {"one", [](){return new DerivedClass1();}}
,   {"two", [](){return new DerivedClass2();}}
};

int main() {
    BaseClass *a = factory["one"](); // Note the function call () at the end
    BaseClass *b = factory["two"]();
    a->foo();
    b->foo();
    delete a;
    delete b;
    return 0;
}

The idea is to make a map that gives you a function that makes an appropriate subclass.

Demo on ideone.

Answer 2

First of all, a syntax primer:

struct Base {
    virtual ~Base() {} // do not forget this if you need polymorphism
};

Then, a "factory" function:

template <typename T>
std::unique_ptr<Base> makeBase() { return std::unique_ptr<Base>(new T{}); }

The type of this function is:

using BaseMaker = std::unique_ptr<Base>(*)();

And finally, putting it altogether:

struct DerivedOne: Base {}; struct DerivedTwo: Base {};

using BaseMakerMap = std::map<std::string, BaseMaker>;

BaseMakerMap const map = { { "DerivedOne", makeBase<DerivedOne> },
                           { "DerivedTwo", makeBase<DerivedTwo> } };

std::unique_ptr<Base> makeFromName(std::string const& n) {
    BaseMakerMap::const_iterator it = map.find(n);

    if (it == map.end()) { return std::unique_ptr<Base>(); } // not found

    BaseMaker maker = it->second;

    return maker();
}

Answer 3

You should be able to do the following:

template<class C>
BaseClass * makeObject<C> () {
    return new C;
}

struct LookupList {
    const char* name;
    BaseClass * (*factoryFunction) ();
};

LookupList list [] = {
    {"ClassOne", &makeObject<DerivedClassOne>},
    {"ClassTwo", &makeObject<DerivedClassTwo>}
};

...

... instantiateFromString ...
    return list[i].factoryFunction ();

I would, however, prefer a Map over an array for the LookupList. In addition, you might want to get familiar with the functional syntax in C++11.