Is it possible to have a non-template class subclass a template class?

Question

I have a template class defined like so:

template <class T>
class Command {
public:
    virtual T HandleSuccess(std::string response) = 0;
    virtual std::string FullCommand() const = 0;
    // ... other methods here ...
};

Will C++ allow me to create a non-template subclass of a template class? What I mean is can I do something like this:

class NoopCommand : public Command<NoopResult> {
public:
    NoopResult HandleSuccess(std::string response);
    std::string FullCommand() const;
    // ... other methods here ...
};

As is that is not working for me because it says the following virtual functions are undefined:

T admix::Command<T>::HandleSuccess(std::string) [with T = admix::NoopResult]
std::string admix::Command<T>::FullCommand() const [with T = admix::NoopResult]

How can I specifically define them for the given T?

Answer 1

The pattern you use is widely known as the "Curiously Recurring Template Pattern". so, yes, you can do that.
I can't think of a reason why it does not compile.

Answer 2

The code you gave compiles for me, without errors (after adding a struct NoopResult { };). Maybe there's a problem in the code you left out?

Answer 3

"virtual functions are undefined" means you have not defined the function bodies of NoopCommand::HandleSuccess and NoopCommand::FullCommand.

The following should solve your problem.

class NoopCommand : public Command<NoopResult> {
public:
    NoopResult HandleSuccess(std::string response) {}
    std::string FullCommand() const {}
    // ... other methods here ...
};

Or you you have a NoopCommand.cpp, make sure it's included in your build process.

Answer 4

litb found the solution on ##c++ last night.

The issue was I was passing a NoopCommand to a function like this:

void SendCommand(Command<T> command);

When I should have made the signature this:

void SendCommand(Command<T>& command);

Making that change allows everything to compile.