I have come across
the need to reorder a variadic list of parameters that is supplied to
the constructor of a struct. After being reordered based on their types,
the parameters will be stored as a tuple. My question is how this can be done so that a modern C++ compiler (e.g. g++-4.7) will not generate unnecessary load or store instructions. That is, when
the constructor is invoked with a list of parameters of variable size, it efficiently pushes each parameter into place based on an ordering over
the parameters' types.
Here is a concrete example. Assume that
the base type of every parameter (without references, rvalue references, pointers, or qualifiers) is either char, int, or float. How can I make it so that
all the parameters of base type char appear first, followed by
all of those of base type int (which leaves
the parameters of base type float last).
The relative order in which
the parameters were given should not be violated within sublists of homogeneous base type.
Example: foo::foo() is called with arguments float a, char&& b, const float& c, int&& d, char e.
The tuple tupe is std::tuple<char, char, int, float, float>, and it is constructed like so: tuple_type{std::move(b), e, std::move(d), a, c}.
Consider
the struct defined below, and assume that
the metafunction deduce_reordered_tuple_type is already implemented. How would you write
the constructor so that it works as intended? If you think that
the code for deduce_reodered_tuple_type, would be useful to you, I can provide it; it's a little long.
template <class... Args> struct
foo
{
// Assume that
the metafunction deduce_reordered_tuple_type is defined.
typedef typename deduce_reordered_tuple_type<Args...>::type tuple_type;
tuple_type t_;
foo(Args&&... args) : t_{reorder_and_forward_parameters<Args>(args)...} {}
};
Edit 1
The technique I describe above does have applications in mathematical frameworks that make heavy use of expression templates, variadic templates, and metaprogramming in order to perform aggressive inlining. Suppose that you wish to define an operator that takes
the product of several expressions, each of which may be passed by reference, reference to const, or rvalue reference. (In my case,
the expressions are conditional probability tables and
the operation is
the factor product, but something like matrix multiplication works suitably as well.)
You need access to
the data provided by each expression in order to evaluate
the product. Consequently, you must move
the expressions passed as rvalue references, copy
the expressions passed by reference to const, and take
the addresses of expressions passed by reference. Using
the technique I describe above now poses several benefits.
Other expressions can use uniform syntax to access data elements from this expression, since
all of
the heavy-lifting metaprogramming work is done beforehand, within
the class.
We can save stack space by grouping
the pointers together and storing
the larger expressions towards
the end of
the tuple.
Implementing certain types of queries becomes much easier (e.g. check whether any of
the pointers stored in
the tuple aliases a given pointer).
Thank you very much for your help!
