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Subject: [boost] [TypeErasure] references
From: Chapman, Alec (archapm_at_[hidden])
Date: 2012-07-26 13:31:43

Hi Steven,

I have a somewhat-generic type erasure class that I would like to switch to yours. The main difficulty I can see is in the way capturing by reference is handled. The approach I take in my class is to pass a boost::reference_wrapper to the constructor:

typedef mpl::vector<copy_constructible<>, type_id_<>, incrementable<> > requirements;
int i = 0;
any<requirements> x(boost::ref(i)); // capture i by reference
++x; // i == 1
any<requirements> y(x); // makes a copy of i
++y; // i == 1, any_cast<int>(y) == 2

My motivation is that I have a bunch of template functions already written that I often call directly, but in other cases I need to compile them separately by instantiating them with any<...>:

---------------------- module 1 -------------------------
// some long algorithm that I don't want to modify to be aware of any<...>
template<class T>
void f(T& t)
    T t2 = t;
    ++t2; // do something with t2

// make a type-erased version for separate compilation
void any_f(any<requirements>& x)
    return f(x);

-------------------- module 2 ----------------------------
// provide a thin wrapper that applies type erasure before calling any_f
template<class T>
void f_fwd(T& t)
    any<requirements> x(boost::ref(t));

// some function that needs to call f
void g(int& i)
    // do something with i...
    // then pass it to f

Is something like this possible with your method? Are there advantages that I'm not seeing to having the type signature contain information about whether the object is held by reference?


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