Get derived value type from std::future::get

I would like to return a std::future from a class method. The value type of the std::future depends on the concrete class. I wonder how I can get() the value of the std::future without knowing a priori which concrete class is used.

Here's a simplified example, which uses only one derived ValueA and AppA class. In my real world problem, I have multiple classes (AppB with ValueB etc) inheriting from the corresponding base classes. I would like to avoid using a templated App class, as in the real world most methods do not depend on the type of ValueBase.

#include <future>
#include <memory>

class ValueBase
{
public:
  virtual int value() const = 0;
};

class ValueA : public ValueBase
{
public:
  int value() const override
  {
    return 0;
  }
};


class AppBase
{
public:
  virtual std::future<const ValueBase&> get() = 0;
};

class AppA : public AppBase
{
public:
  std::future<const ValueBase&> get() override
  {
    std::promise<const ValueBase&> promise;
    promise.set_value(value_);
    return promise.get_future();
  }

private:
  ValueA value_;
};


int main()
{
  std::unique_ptr<AppBase> app = std::make_unique<AppA>();
  auto future = app->get();

  // If I know that app uses ValueA, I can cast it
  const auto valueA = dynamic_cast<const ValueA&>(future.get());

  // How to get the value if the type/concrete class is not known?
  // The following line does not compile with
  // Variable type 'const ValueBase' is an abstract class
  const auto value = future.get();
}