problem_29.md

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Problem 29: Logging

2017-11-23

We want to encapsulate logging functionality and "add" it to any class.

template<typename T, typename Data> class Logger {
    public:
        void loggedSet(Data x) {
            std::cout << "setting data to " << x << std::endl;
            static_cast<T*>(this)->set(x);  // No virtual call overhead
        }
};

class Box: public Logger<Box, int> {
        friend class Logger<Box, int>;
        int x;
        void set(int y) { x = -y; }
    public:
        Box(): x{0} { loggedSet(0); }
};

Box b;
b.loggedSet(1);
b.loggedSet(4);
// etc.

Another approach:

class Box {
        int x;
    public:
        Box(): x{0} {}
        void set(int y) { x = y ;}
};

                                    // Mixin Inheritance
template<typename T, typename Data> class Logger: public T {
    public:
        void loggedSet(Data x) {
            std::cout << "setting to" << x << std::endl;
            set(x); // No vtable overhead
        } 
};

using BoxLogger = Logger<Box, int>;
Boxlogger b;
b.loggedSet(1);
b.loggedSet(4);
//etc.

Mixins - can mix and match subclass functionality without writing new classes

Note: if SpecialBox is a subclass of Box, then SpecialBox has no relation to Logger<Box, int>. Nor is there any relationship between Logger<SpecialBox, int>, Logger<Box, int>.

But with CRTP, SpecialBox is a subtype of Logger<Box, int>

• Can specialize behaviour of virtual functions

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