Extension methods in c++

Question

I was searching for an implementation of extension methods in c++ and came upon this comp.std.c++ discussion which mentions that polymorphic_map can be used to

Answer 1

You can enable kinda extension methods for your own class/struct or for some specific type in some scope. See rough solution below.

class Extensible
{
public:
    template
    std::function operator|
        (std::function& extension)
    {
        return [this, &extension](Args... args) -> TRes
        {
            return extension(*static_cast(this), std::forward(args)...);
        };
    }
};

Then inherit your class from this and use like

class SomeExtensible : public Extensible { /*...*/ };
std::function fn;
SomeExtensible se;
int i = (se | fn)(4);

Or you can declare this operator in cpp file or namespace.

//for std::string, for example
template
std::function operator|
    (std::string& s, std::function& extension)
{
    return [&s, &extension](Args... args) -> TRes
    {
        return extension(s, std::forward(args)...);
    };
}

std::string s = "newStr";
std::function init = [](std::string& s) {
    return s = "initialized";
};
(s | init)();

Or even wrap it in macro (I know, it's generally bad idea, nevertheless you can):

#define ENABLE_EXTENSIONS_FOR(x) \
template \
std::function operator| (x s, std::function& extension) \
{ \
    return [&s, &extension](Args... args) -> TRes \
    { \
        return extension(s, std::forward(args)...); \
    }; \
}

ENABLE_EXTENSIONS_FOR(std::vector&);