Polymorphic logger (virtual templated functions?)

Question

CONTEXT

I have a library used by several software. Some are basic command line and some have a Qt UI.

I want to implement a unique log class inside this library, so every software can use it. However, if we are in a Qt environment, I'd like to use the qDebug() function with a specific QtMessageHandler . The goal is to be able to log errors inside this library, and the logs will be printed differently, depending on if the library is used in a UI environment or not (the library has no dependencies to Qt).

---

WHAT I WANT

Globally, I want something like this:

class Logger
{
public:
    template <class T>
    void log(const T& tolog) { handler.log(tolog); }

    void setHandler(HANDLER???& h) { handler = h; }
    const HANDLER???& getHandler() const { return handler; }

private:
    HANDLER??? handler;
}

With the handler that will be, for command line software, something very simple like:

class CLHandler
{
public:
    template <class T>
    void log(const T& tolog) { out << tolog << std::endl; }

private:
    std::ostream out;
}

and for UI, I want to use qDebug() so I can setup a custom QtMessageHandler to print the error in the UI, and log it in a file:

class UIHandler
{
public:
    template <class T>
    void log(const T& tolog) { qDebug() << tolog; }
}

---

PROBLEM

As you can see, the problem is in the class Logger : what type will be the handler?

I can't really create an interface because of virtual template functions:

class IHandler
{
public:
    virtual ~IHandler() = default;

    template <class T>
    virtual void log(const T& tolog) = 0; // ERROR templated virtual function!
}

---

Need help

I want the IHandler::tolog function to be templated because I want to use the power of the operator<< for both ostream and qDebug() . And I don't want to reimplement all the overloads myself (long list for ostream , even longer for qDebug !).

I want to achieve it, no matter how (lambda functions with auto ?)... Any suggestions welcome (I may do something wrong here?).

Thanks :)

Answer 1

It's obviously not possible to have templated virtual functions, but you could use type erasure to "erase" the concrete type, so you don't need a template anymore.

Basically, you create an interface ILoggableValue that knows how to log your values both using QDebug and std::ostream streams, and use templates to generate concrete implementations for different types:

class ILoggableValue {
public:
    virtual ~ILoggableValue() = default;
    virtual void log(QDebug &os) const = 0;
    virtual void log(std::ostream &os) const = 0;
};

template <typename T>
class LoggableValue : public ILoggableValue {
public:
    LoggableValue(const T &value) : value{value} {}
    void log(QDebug &os) const override {
        // implementation of log for QDebug goes here
        os << value;
    }
    void log (std::ostream &os) const override {
        // implementation of log for std::ostream goes here
        os << value << std::endl;
    }
private:
    const T &value;
};

Then, you create IHandler the same way you suggested it, but now you can use ILoggableValue to erase the template:

class IHandler {
public:
    virtual ~IHandler() = default;
    virtual void log(const ILoggableValue &tolog) const = 0;
};

class CLHandler : public IHandler {
public:
    explicit CLHandler(std::ostream &out) : out{out} {} 
    void log(const ILoggableValue &tolog) const override {
        tolog.log(out);
    }
private:
    std::ostream &out;
};

class UIHandler : public IHandler {
public:
    void log(const ILoggableValue &tolog) const override {
        tolog.log(qDebug());
    }
};

Finally, you use IHandler in your Logger :

class Logger {
public:
    Logger(std::unique_ptr<IHandler> h) : handler(std::move(h)) {}
    template <class T>
    void log(const T& tolog) { handler->log(LoggableValue<T>(tolog)); }

    void setHandler(std::unique_ptr<IHandler> &h) { handler = std::move(h); }
    const IHandler &getHandler() const { return *handler; }

private:
    std::unique_ptr<IHandler> handler;
};

Here is a live example.