Choose C++ template at runtime - c++

Question

Is there any way to achieve the functionality of below code without creating the mapping between strings and classes manually?
template<class base, typename T>
base* f(const std::string &type, T &c) {
if(type == "ClassA") return new ClassA(c);
else if(type == "ClassB") return new ClassB(c);
// many more else if...
return nullptr;
}
All classes looks something like this:
class ClassA: public BaseClass {
public:
std::string label="ClassA";
...
};
And we can use it as:
BaseClass *b = f<BaseClass>("ClassA", DifferentObject);
Each new class results in a new if else line of code. Is there any way to automate this so f function "updates" itself when new supported class is added? The solution must work for C++11.

Answer 1

A possible macro:
#include <memory>
#include <string>
class BaseClass {};
class ClassA : public BaseClass {
public:
std::string label = "ClassA";
explicit ClassA(int /*unused*/) {}
};
class ClassB : public BaseClass {
public:
std::string label = "ClassB";
explicit ClassB(int /*unused*/) {}
};
template<class base, typename T>
auto f(const std::string &type, T c) -> std::unique_ptr<base> {
#define CASE(NAME) \
if (type == "NAME") { \
return std::unique_ptr<base>(new NAME(c)); \
}
CASE(ClassA)
CASE(ClassB)
//...
#undef CASE
return nullptr; // Statement at the end needed for last else!
}
auto main() -> int {
auto b = f<BaseClass>("ClassA", 0);
}
Also use unique_ptr since memory managing raw pointers are EVIL.

Answer 2

In that case if the class name is equal to the string, you can simplify your code with the following macro:
#define STRING_TO_CLASS (className) if(type == "className") return new className(c);
template<class base, typename T>
base* f(const std::string &type, T &c) {
STRING_TO_CLASS(ClassA)
STRING_TO_CLASS(ClassB)
return nullptr;
}
Personally I hate macros, but it disturbs only me. However, at compile time, the following code wil be generated, after the macros are resolved.
template<class base, typename T>
base* f(const std::string &type, T &c) {
if(type == "ClassA") return new ClassA(c);
if(type == "ClassB") return new ClassB(c);
return nullptr;
}
As you see, in the end only the else keyword is removed. Also, you need to modify your code if a new class is added.

Answer 3

You could use the registry pattern like this:
#include <map>
#include <functional>
#include <string>
template< typename T, typename X >
using Factory = std::function< T* ( X& ) >;
template< typename Base, typename X >
struct Registry {
using Map = std::map<std::string,Factory<Base,X> >;
static Map registry;
template< typename T >
struct Register {
Register( const std::string& name ) {
registry[ name ] = []( X& x ) -> T* { return new T(x); };
}
};
};
template< typename Base, typename X >
Base* factory(const std::string &type, X &c ) {
auto it = Registry<Base,X>::registry.find( type );
if ( it!=Registry<Base,X>::registry.end() ) {
return (it->second)(c);
}
return nullptr;
}
struct X {};
struct A {
A( X& x ) {};
virtual ~A() {}
};
struct B : public A {
B( X& x ) : A(x) {};
};
struct C : public A {
C( X& x ) : A(x) {};
};
struct D : public B {
D( X& x ) : B(x) {};
};
// Register class
template<> Registry<A,X>::Map Registry<A,X>::registry{};
Registry<A,X>::Register<B> regB( "B" );
Registry<A,X>::Register<C> regC( "C" );
Registry<A,X>::Register<D> regD( "D" );
#include <iostream>
int main() {
X x;
A* ptr = factory<A,X>( "B", x );
B* bptr = dynamic_cast<B*>( ptr );
if ( bptr!= nullptr ) {
std::cout << "Success!" << std::endl;
return 0;
}
std::cout << "Failed!" << std::endl;
return 1;
}

Answer 4

The correct pattern to use here is the "Abstract Factory" pattern.
Maybe you can look it up.
To give you and idea (and not more), what is possible, I will show you the below code, which even accepts constructors with different signature.
#include <iostream>
#include <map>
#include <utility>
#include <any>
// Some demo classes ----------------------------------------------------------------------------------
struct Base {
Base(int d) : data(d) {};
virtual ~Base() { std::cout << "Destructor Base\n"; }
virtual void print() { std::cout << "Print Base\n"; }
int data{};
};
struct Child1 : public Base {
Child1(int d, std::string s) : Base(d) { std::cout << "Constructor Child1 " << d << " " << s << "\n"; }
virtual ~Child1() { std::cout << "Destructor Child1\n"; }
virtual void print() { std::cout << "Print Child1: " << data << "\n"; }
};
struct Child2 : public Base {
Child2(int d, char c, long l) : Base(d) { std::cout << "Constructor Child2 " << d << " " << c << " " << l << "\n"; }
virtual ~Child2() { std::cout << "Destructor Child2\n"; }
virtual void print() { std::cout << "Print Child2: " << data << "\n"; }
};
struct Child3 : public Base {
Child3(int d, long l, char c, std::string s) : Base(d) { std::cout << "Constructor Child3 " << d << " " << l << " " << c << " " << s << "\n"; }
virtual ~Child3() { std::cout << "Destructor Child3\n"; }
virtual void print() { std::cout << "Print Child3: " << data << "\n"; }
};
using UPTRB = std::unique_ptr<Base>;
template <class Child, typename ...Args>
UPTRB createClass(Args...args) { return std::make_unique<Child>(args...); }
// The Factory ----------------------------------------------------------------------------------------
template <class Key, class Object>
class Factory
{
std::map<Key, std::any> selector;
public:
Factory() : selector() {}
Factory(std::initializer_list<std::pair<const Key, std::any>> il) : selector(il) {}
template<typename Function>
void add(Key key, Function&& someFunction) { selector[key] = std::any(someFunction); };
template <typename ... Args>
Object create(Key key, Args ... args) {
if (selector.find(key) != selector.end()) {
return std::any_cast<std::add_pointer_t<Object(Args ...)>>(selector[key])(args...);
}
else return nullptr;
}
};
int main()
{
// Define the factory with an initializer list
Factory<int, UPTRB> factory{
{1, createClass<Child1, int, std::string>},
{2, createClass<Child2, int, char, long>}
};
// Add a new entry for the factory
factory.add(3, createClass<Child3, int, long, char, std::string>);
// Some test values
std::string s1(" Hello1 "); std::string s3(" Hello3 ");
int i = 1; const int ci = 1; int& ri = i; const int& cri = i; int&& rri = 1;
UPTRB b1 = factory.create(1, 1, s1);
UPTRB b2 = factory.create(2, 2, '2', 2L);
UPTRB b3 = factory.create(3, 3, 3L, '3', s3);
b1->print();
b2->print();
b3->print();
b1 = factory.create(2, 4, '4', 4L);
b1->print();
return 0;
}