Resolve enum template parameters at run-time

Question

I am trying to come up with a piece of logic that takes multiple enums and determine a struct type and its corresponding size to pass to a third-party component, and I am consid

Answer 1

You cant do this at run-time using templates, but you do have some other options you can use localise the changes.

My preference would be to use polymorphism:

class Foo {
  static Foo* createFoo(FOO_TYPE type) {
  switch(type) {
     case FOO_TYPE_0:
       return new FooType0();
     case FOO_TYPE_1:
       return new FooType1();
  }
  virtual void callFunction(ThridParty thirdParty)=0;
}

class FooType0 : public Foo {
   void callFunction(ThridParty thirdParty) {
      FOO_0 Foo = {};
      UINT size = sizeof(Foo);
      m_thirdParty->CallFunction(&Foo, size);
   } 
}

class FooType1 : public Foo {
   void callFunction(ThridParty thirdParty) {
      FOO_1 Foo = {};
      UINT size = sizeof(Foo);
      m_thirdParty->CallFunction(&Foo, size);
   } 
}

Your switch block would then become:

// Note this leaks ... see below for non-leaking version.
createFoo(m_foo)->callFunction(m_thirdParty);

Now there's some ugly duplication in each of the FooTypeX classes, so you can remove that with templatization:

class Foo {
  static Foo* createFoo(FOO_TYPE type) {
  switch(type) {
     case FOO_TYPE_0:
       return new FooX<FOO_1>();
     case FOO_TYPE_1:
       return new FooX<FOO_2>();
  }
  virtual void callFunction(ThridParty thirdParty)=0;
}

class FooX<BaseStruct> : public Foo {
   void callFunction(ThridParty thirdParty) {
      BaseStruct foo = {};
      UINT size = sizeof(foo);
      m_thirdParty->CallFunction(&foo, size);
   } 
} 

// A bit of a hack to prevent leaking. In real code you may want to 
// handle ownership differently.
std::unique_ptr<Foo>(createFoo(m_foo))->callFunction(m_thirdParty);