Question

I have several subclasses of one superclass and want to create an instance of a specific class according to a given string

Superclass instantiateSubclass(string s);

Instead of having a huge if-cascade I want to work with a config file to accomplish this.

This allows me to change the possible values for string s without recompiling and I hope it leads to a more concise code.

The config file should contain strings like “subclass1”, “subclass2”, but how do I create the class according to a string then?

Basically I need a mapping from string to class, is this possible in C++? I think other languages offer possibilities like reflection for this problem.

Answer 1

Register your classes:

struct Base;

struct Derived1 : Base
{
    static Base * create() { return new Derived1; }
};

std::map<std::string, Base * (*)()> registry = { {"derived1", &Derived1::create},
                                                 /* ... */
                                               };

To make:

Base * create_from_string(std::string const & s)
{
    auto it = registry.find(s);
    return it == registry.end() ? nullptr : (it->second)();
}