Template class casting in constructor - c++

Lets say I have a class like so:
template< typename T, int nDimensions = 2 >
class Vec
{
private:
std::array< T, nDimensions > elements_;
}
Then I typedef out a few different types.
typedef Vec< int, 2 > Vec2i;
typedef Vec< int, 3 > Vec3i;
typedef Vec< float, 2 > Vec2f;
typedef Vec< float, 3 > Vec3f;
What would the constructor be if I wanted to convert from one type to another?
Vec2i something(10,20); //10,20
Vec2f somethingElse(something); //10.0f,20.0f
Same goes for different sizes:
Vec3f somethingmore(something); //10.0f,20.0f,0.0f
So far I have:
template<typename F>
Vec(const F& other)
{
for (int i = 0; i < nDimensions; i++)
{
this->elements_[i] = static_cast<F>(other[i]); //I know this is wrong.
}
}
I cant figure out a good way to get the base type of the other class to do the static casts on each element, nor a good way to get the others nDimension size so I can do proper bounds checking.

What would the constructor be if I wanted to convert from one type to another?
The most generic constructor would be:
template <typename T2, int nDimension2>
Vec(Vec<T2, nDimension2> const& copy) { ... }
That will need appropriate logic to make sure that you don't access memory using out of bounds indices.
A less generic constructor would be:
template <typename T2>
Vec(Vec<T2, nDimension> const& copy) { ... }
Here, you can use std::copy to copy the elements.
template <typename T2>
Vec(Vec<T2, nDimension> const& copy) { std::copy(copy.elements_.begin(),
copy.elements_.ennd(),
this->elements_.begin()); }

Related

Using the allocator for a structure fields of different types

I have a custom allocator which instances share a memory buffer of type TrivialSpace:
class TrivialSpace
{
public:
uint8_t * Allocate(std::size_t memory_size)
{
...
return ptr_to_mem;
}
...
};
template <class T>
class TrivialAllocator
{
public:
using value_type = T;
TrivialAllocator(TrivialSpace & space) : m_space(space)
{
}
template <class Q>
TrivialAllocator(const TrivialAllocator<Q> & other) : m_space(other.m_space)
{
}
T * allocate(std::size_t n)
{
return reinterpret_cast<T*>(m_space.Allocate(n * sizeof(T)));
}
void deallocate(T* p, std::size_t n)
{
}
private:
TrivialSpace & m_space;
template <class Q>
friend class TestAllocator;
};
and a structure containing std::string and std::vector:
using String = std::basic_string<char, std::char_traits<char>, TrivialAllocator<char>>;
template <class T>
using Vector = std::vector<T, TrivialAllocator<T>>;
struct A
{
String a;
Vector<int> b;
Vector<String> c;
};
1) what is the right way to create an instance of A structure my TrivialAllocator?
2) And how to make the code work with both TrivialAllocator and std::allocator? When the field types are defined as above or as follows (depending on some #ifdef probably):
using String = std::string;
template <class T>
using Vector = std::vector<T>;
My idea was to pass the instance of Allocator to the structure constructor, but it is not clear what specialization of Allocator should I pass - Allocator<char>, Allocator<int> or something else.
I would specifically take 3 allocators (for example, if you used a polymorphic allocator, you might want a specific one for each object).
You could also have an overloaded constructor that takes one allocator and constructs the other three from it (Only if each allocator is rebound from the other 2, which in this case or with a polymorphic allocator they are)
A(Allocator<char> a_alloc, Allocator<int> b_alloc, Allocator<String> c_alloc) : a(a_alloc), b(b_alloc), c(c_alloc) {}
template<class Alloc,
std::enable_if_t<
std::is_same_v<std::allocator_traits<Alloc>::template rebind_alloc<char>, Allocator<char>> &&
// Next 2 maybe redundant based on what you are doing
std::is_same_v<std::allocator_traits<Alloc>::template rebind_alloc<int>, Allocator<int>> &&
std::is_same_v<std::allocator_traits<Alloc>::template rebind_alloc<String>, Allocator<String>>,
int> = 0>
A(Alloc alloc) : A(Allocator<char>(alloc), Allocator<int>(alloc), Allocator<String>(alloc)) {}
Or you could pass the issue to the constructor of the class and move from a string and two vectors (Or leave it as an aggregate and initialize with allocators where the type is used)

Nesting variadic template

I have two classes that utilize initialize lists. One is a Vector styled class that hold a list of values.
Vec.h:
template< typename T, int nDimensions = 2 >
class Vec{
private:
std::array< T, nDimensions > elements_;
public:
template <typename... U>
Vec(U... ts) : elements_{ ts... } {}
}
Then when used:
typedef Vec< int, 2 > Vec2i;
Vec2i twoi = { 1,2 };
Vec2i twoi2 = twoi; //I have copy constructor and other methods finished.
This class works all fine and dandy, however when I try to use this class with my Matrix styled class, I cant figure out the correct syntax for its constructor.
template< typename T, int X,int Y>
class Mat {
private:
std::array< Vec<T, X>, Y > elements_;
}
Id like to use it like so:
typedef Mat<int, 3,3> Mat3i;
Mat3i threemat = { {1,2,3},
{4,5,6},
{7,8,9}};
Now, iv tried using initializer lists as the constructor with some success, but I cant figure out the syntax to pass the sub lists along.
Mat(std::initializer_list<Vec<T, X>> values) {
for (auto& t : values) {
//What goes here?
}
}
Iv also tried iteration over the list and assigning them manually, but thats a no go.
Its also worth noting that its important that these classes have consecutive chunks of memory for there lists, and no other variables. Otherwise id be using other types instead of std::array. (For casting and union purposes.)
I'm debating if I need to reinterpret cast each value as a Vec then copy over the values.
As alternative:
Mat(std::initializer_list<Vec<T, X>> values) {
std::copy(values.begin(), values.end(), elements_.begin());
}
I'm an idiot.
Mat(std::initializer_list<Vec<T, X>> values) {
int i = 0;
for (auto& t : values) {
elements_[i++] = t;
}
}

container of template class in c++

I have a template, which may be used with different stl map (map<int, int>, map<int, char>, etc.), as follows.
template <typename Map> struct TriMaps
{
Map& next;
Map& prev;
Map& curr;
};
so, TriMaps<init, int> intTriMaps; TriMaps<int, char> charTriMaps;
Then, is it possible to have a container of the above TriMaps, in different type? e.g.
vector <TriMaps> vecMaps;
which contain intTriMaps and charTriMaps? Usually container requires the same type. But I really need a container for my case. No boost or third library available.
First determine and finalize what this class template TriMaps is expected to do. In C++, you must initialize a reference to something. You probably need a non-reference type (or a pointer type). Consider:
template <typename Map>
struct TriMaps
{
Map next;
Map prev;
Map curr;
};
Usage:
TriMaps<int> IntMaps;
vector<TriMaps<int>> iv;
If you need two template type argument, you can do this way:
template <typename MapType1, typename MapType2>
struct TriMaps
{
MapType1 a;
Maptype2 b;
};
Usage:
TriMaps<int, float> IntMaps;
vector<TriMaps<int, float>> iv;
EDIT:
Carefully understand that following simple class will not compile.
class TriMap
{
int & ref_int;
};
One approach is to refer a global variable (just to make it compile), or take an int& via a constructor.
// Approach 1
int global;
class TriMap
{
int & ref_int;
public:
TriMap() : ref_int(global) {}
};
// Approach 2
class TriMap
{
int & ref_int;
public:
TriMap(int & outer) : ref_int(outer) {}
};
You can't have a std::vector of different type
but if you can have base class
struct TriMapBase { virtual ~TriMapBase() = default; };
template <typename Map> struct TriMaps : TriMapBase { /* Your implementation */ };
Then you may have:
TriMapBase<std::map<int, int>> mapint;
TriMapBase<std::map<int, char>> mapchar;
std::vector<TriMapBase*> v = {&mapint, &mapchar};
but you have to use virtual functions
or dynamic_cast to retrieve the correct type afterward...
or use

How can I use C++11 variadic templates to define a vector-of-tuples backed by a tuple-of-vectors?

Suppose I have a bunch of vectors:
vector<int> v1;
vector<double> v2;
vector<int> v3;
all of the same length. Now, for every index i, I would like to be able to treat (v1[i], v2[i], v3[i]) as a tuple, and maybe pass it around. In fact, I want to have a a vector-of-tuples rather than a tuple-of-vectors, using which I can do the above. (In C terms, I might say an array-of-structs rather than a struct-of-arrays). I do not want to effect any data reordering (think: really long vectors), i.e. the new vector is backed by the individual vectors I pass in. Let's .
Now, I want the class I write (call it ToVBackedVoT for lack of a better name) to support any arbitrary choice of vectors to back it (not just 3, not int, double and int, not every just scalars). I want the vector-of-tuples to be mutable, and for no copies to be made on construction/assignments.
If I understand correctly, variadic templates and the new std::tuple type in C++11 are the means for doing this (assuming I don't want untyped void* arrays and such). However, I only barely know them and have never worked with them. Can you help me sketch out how such a class will look like? Or how, given
template <typename ... Ts>
I can express something like "the list of template arguments being the replacement of each typename in the original template arguments with a vector of elements of this type"?
Note: I think I might also want to later be able to adjoin additional vectors to the backing vectors, making an instance of ToVBackedVoT<int, double, int> into, say, an instance of ToVBackedVoT<int, double, int, unsigned int>. So, bear that in mind when answering. This is not critically important though.
One idea is to keep the storage in the "struct of array" style in form of vectors for good performance if only a subset of the fields are used for a particular task. Then, for each kind of task requiring a different set of fields, you can write a lightweight wrapper around some of those vectors, giving you a nice random access iterator interface similar to what std::vector supports.
Concerning the syntax of variadic templates, this is how a wrapper class (without any iterators yet) could look like:
template<class ...Ts> // Element types
class WrapMultiVector
{
// references to vectors in a TUPLE
std::tuple<std::vector<Ts>&...> m_vectors;
public:
// references to vectors in multiple arguments
WrapMultiVector(std::vector<Ts> & ...vectors)
: m_vectors(vectors...) // construct tuple from multiple args.
{}
};
To construct such a templated class, it's often preferred to have a template type deducting helper function available (similar to those make_{pair|tuple|...} functions in std):
template<class ...Ts> // Element types
WrapMultiVector<Ts...> makeWrapper(std::vector<Ts> & ...vectors) {
return WrapMultiVector<Ts...>(vectors...);
}
You already see different types of "unpacking" the type list.
Adding iterators suitable to your application (you requested in particular random access iterators) is not so easy. A start could be forward only iterators, which you might extend to random access iterators.
The following iterator class is capable of being constructed using a tuple of element iterators, being incremented and being dereferenced to obtain a tuple of element references (important for read-write access).
class iterator {
std::tuple<typename std::vector<Ts>::iterator...> m_elemIterators;
public:
iterator(std::tuple<typename std::vector<Ts>::iterator...> elemIterators)
: m_elemIterators(elemIterators)
{}
bool operator==(const iterator &o) const {
return std::get<0>(m_elemIterators) == std::get<0>(o.m_elemIterators);
}
bool operator!=(const iterator &o) const {
return std::get<0>(m_elemIterators) != std::get<0>(o.m_elemIterators);
}
iterator& operator ++() {
tupleIncrement(m_elemIterators);
return *this;
}
iterator operator ++(int) {
iterator old = *this;
tupleIncrement(m_elemIterators);
return old;
}
std::tuple<Ts&...> operator*() {
return getElements(IndexList());
}
private:
template<size_t ...Is>
std::tuple<Ts&...> getElements(index_list<Is...>) {
return std::tie(*std::get<Is>(m_elemIterators)...);
}
};
For demonstration purposes, two different patterns are in this code which "iterate" over a tuple in order to apply some operation or construct a new tuple with some epxression to be called per element. I used both in order to demonstrate alternatives; you can also use the second method only.
tupleIncrement: You can use a helper function which uses meta programming to index a single entry and advance the index by one, then calling a recursive function, until the index is at the end of the tuple (then there is a special case implementation which is triggered using SFINAE). The function is defined outside of the class and not above; here is its code:
template<std::size_t I = 0, typename ...Ts>
inline typename std::enable_if<I == sizeof...(Ts), void>::type
tupleIncrement(std::tuple<Ts...> &tup)
{ }
template<std::size_t I = 0, typename ...Ts>
inline typename std::enable_if<I < sizeof...(Ts), void>::type
tupleIncrement(std::tuple<Ts...> &tup)
{
++std::get<I>(tup);
tupleIncrement<I + 1, Ts...>(tup);
}
This method can't be used to assign a tuple of references in the case of operator* because such a tuple has to be initialized with references immediately, which is not possible with this method. So we need something else for operator*:
getElements: This version uses an index list (https://stackoverflow.com/a/15036110/592323) which gets expanded too and then you can use std::get with the index list to expand full expressions. The IndexList when calling the function instantiates an appropriate index list which is only required for template type deduction in order to get those Is.... The type can be defined in the wrapper class:
// list of indices
typedef decltype(index_range<0, sizeof...(Ts)>()) IndexList;
More complete code with a little example can be found here: http://ideone.com/O3CPTq
Open problems are:
If the vectors have different sizes, the code fails. Better would be to check all "end" iterators for equality; if one iterator is "at end", we're also "at end"; but this would require some logic more than operator== and operator!= unless it's ok to "fake" it in; meaning that operator!= could return false as soon as any operator is unequal.
The solution is not const-correct, e.g. there is no const_iterator.
Appending, inserting etc. is not possible. The wrapper class could add some insert or and / or push_back function in order to make it work similar to std::vector. If your goal is that it's syntactically compatible to a vector of tuples, reimplement all those relevant functions from std::vector.
Not enough tests ;)
An alternative to all the variadic template juggling is to use the boost::zip_iterator for this purpose. For example (untested):
std::vector<int> ia;
std::vector<double> d;
std::vector<int> ib;
std::for_each(
boost::make_zip_iterator(
boost::make_tuple(ia.begin(), d.begin(), ib.begin())
),
boost::make_zip_iterator(
boost::make_tuple(ia.end(), d.end(), ib.end())
),
handle_each()
);
Where your handler, looks like:
struct handle_each :
public std::unary_function<const boost::tuple<const int&, const double&, const int&>&, void>
{
void operator()(const boost::tuple<const int&, const double&, const int&>& t) const
{
// Now you have a tuple of the three values across the vector...
}
};
As you can see, it's pretty trivial to expand this to support an arbitrary set of vectors..
From asker's clarification on how this would be used (code that takes a tuple), I'm going to propose this instead.
//give the i'th element of each vector
template<typename... Ts>
inline tuple<Ts&...> ith(size_t i, vector<Ts>&... vs){
return std::tie(vs[i]...);
}
There's a proposal to allow parameter packs to be saved as members of classes (N3728). Using that, here's some untested and untestable code.
template<typename... Types>
class View{
private:
vector<Types>&... inner;
public:
typedef tuple<Types&...> reference;
View(vector<Types>&... t): inner(t...) {}
//return smallest size
size_t size() const{
//not sure if ... works with initializer lists
return min({inner.size()...});
}
reference operator[](size_t i){
return std::tie(inner[i]...);
}
};
And iteration:
public:
iterator begin(){
return iterator(inner.begin()...);
}
iterator end(){
return iterator(inner.end()...);
}
//for .begin() and .end(), so that ranged-based for can be used
class iterator{
vector<Types>::iterator... ps;
iterator(vector<Types>::iterator... its):ps(its){}
friend View;
public:
//pre:
iterator operator++(){
//not sure if this is allowed.
++ps...;
//use this if not:
// template<typename...Types> void dummy(Types... args){} //global
// dummy(++ps...);
return *this;
}
iterator& operator--();
//post:
iterator operator++(int);
iterator operator--(int);
//dereference:
reference operator*()const{
return std::tie(*ps...);
}
//random access:
iterator operator+(size_t i) const;
iterator operator-(size_t i) const;
//need to be able to check end
bool operator==(iterator other) const{
return std::make_tuple(ps...) == std::make_tuple(other.ps...);
}
bool operator!=(iterator other) const{
return std::make_tuple(ps...) != std::make_tuple(other.ps...);
}
};
You may use something like:
#if 1 // Not available in C++11, so write our own
// class used to be able to use std::get<Is>(tuple)...
template<int... Is>
struct index_sequence { };
// generator of index_sequence<Is>
template<int N, int... Is>
struct make_index_sequence : make_index_sequence<N - 1, N - 1, Is...> { };
template<int... Is>
struct make_index_sequence<0, Is...> : index_sequence<Is...> { };
#endif
// The 'converting' class
// Note that it doesn't check that vector size are equal...
template<typename ...Ts>
class ToVBackedVoT
{
public:
explicit ToVBackedVoT(std::vector<Ts>&... vectors) : data(vectors...) {}
std::tuple<const Ts&...> operator [] (unsigned int index) const
{
return at(index, make_index_sequence<sizeof...(Ts)>());
}
std::tuple<Ts&...> operator [] (unsigned int index)
{
return at(index, make_index_sequence<sizeof...(Ts)>());
}
private:
template <int... Is>
std::tuple<const Ts&...> at(unsigned int index, index_sequence<Is...>) const
{
return std::tie(std::get<Is>(data)[index]...);
}
template <int... Is>
std::tuple<Ts&...> at(unsigned int index, index_sequence<Is...>)
{
return std::tie(std::get<Is>(data)[index]...);
}
private:
std::tuple<std::vector<Ts>&...> data;
};
And to iterate, create an 'IndexIterator' like the one in https://stackoverflow.com/a/20272955/2684539
To adjoin additional vectors, you have to create an other ToVBackedVoT as std::tuple_cat does for std::tuple
Conversion to a std::tuple of vectors (vector::iterators):
#include <iostream>
#include <vector>
// identity
// ========
struct identity
{
template <typename T>
struct apply {
typedef T type;
};
};
// concat_operation
// ================
template <typename Operator, typename ...> struct concat_operation;
template <
typename Operator,
typename ...Types,
typename T>
struct concat_operation<Operator, std::tuple<Types...>, T>
{
private:
typedef typename Operator::template apply<T>::type concat_type;
public:
typedef std::tuple<Types..., concat_type> type;
};
template <
typename Operator,
typename ...Types,
typename T,
typename ...U>
struct concat_operation<Operator, std::tuple<Types...>, T, U...>
{
private:
typedef typename Operator::template apply<T>::type concat_type;
public:
typedef typename concat_operation<
Operator,
std::tuple<Types..., concat_type>,
U...>
::type type;
};
template <
typename Operator,
typename T,
typename ...U>
struct concat_operation<Operator, T, U...>
{
private:
typedef typename Operator::template apply<T>::type concat_type;
public:
typedef typename concat_operation<
Operator,
std::tuple<concat_type>,
U...>
::type type;
};
// ToVectors (ToVBackedVoT)
// =========
template <typename ...T>
struct ToVectors
{
private:
struct to_vector {
template <typename V>
struct apply {
typedef typename std::vector<V> type;
};
};
public:
typedef typename concat_operation<to_vector, T...>::type type;
};
// ToIterators
// ===========
template <typename ...T>
struct ToIterators;
template <typename ...T>
struct ToIterators<std::tuple<T...>>
{
private:
struct to_iterator {
template <typename V>
struct apply {
typedef typename V::iterator type;
};
};
public:
typedef typename concat_operation<to_iterator, T...>::type type;
};
int main() {
typedef ToVectors<int, double, float>::type Vectors;
typedef ToVectors<Vectors, int, char, bool>::type MoreVectors;
typedef ToIterators<Vectors>::type Iterators;
// LOG_TYPE(Vectors);
// std::tuple<
// std::vector<int, std::allocator<int> >,
// std::vector<double, std::allocator<double> >,
// std::vector<float, std::allocator<float> > >
// LOG_TYPE(Iterators);
// std::tuple<
// __gnu_cxx::__normal_iterator<int*, std::vector<int, std::allocator<int> > >,
// __gnu_cxx::__normal_iterator<double*, std::vector<double, std::allocator<double> > >,
// __gnu_cxx::__normal_iterator<float*, std::vector<float, std::allocator<float> > > >
}
As an alternative similar to boost::zip_iterator I wrote a zip function with a very simple interface:
vector<int> v1;
vector<double> v2;
vector<int> v3;
auto vec_of_tuples = zip(v1, v2, v3);
For example, iterate over these tuples:
for (auto tuple : zip(v1, v2, v3)) {
int x1; double x2; int x3;
std::tie(x1, x2, x3) = tuple;
//...
}
Here, zip() takes any number of ranges of any type. It returns an adaptor which can be seen as a lazily evaluated range over a tuple of elements originating from the wrapped ranges.
The adaptor is part of my Haskell-style functional library "fn" and implemented using variadic templates.
Currently it doesn't support modification of the original ranges' values via the adaptor because of the design of the library (it's intended to be used with non-mutable ranges like in functional programming).
A brief explanation on how this is done is: zip(...) returns an adaptor object which implements begin() and end(), returning an iterator object. The iterator holds a tuple of iterators to the wrapped ranges. Incrementing the iterator increments all wrapped iterators (which is implemented using an index list and unpacking an incrementing expression into a series of expressions: ++std::get<I>(iterators)...). Dereferencing the iterator will decrement all wrapped iterators and pass it to std::make_tuple (which is also implemented as unpacking the expression *std::get<I>(iterators)...).
P.S. Its implementation is based on a lot of ideas coming from answers to this question.

boost.proto + unwrap expression from domain-specific expression wrapper

Background question: boost.proto + modify expression tree in place
Hi, consider the following transform to extract the value_type from a vector_expr (see previous questions)
template <class T> struct value_type_trait;
template <std::size_t D, class T>
struct value_type_trait<vector<D, T> >
{
typedef typename vector<D, T>::value_type type;
};
struct deduce_value_type
: proto::or_<
proto::when <vector_terminal, value_type_trait<proto::_value>() >
, proto::when <scalar_terminal, proto::_value>
, proto::otherwise <
proto::_default<deduce_value_type>()
>
>
{};
The above code can be used to provide 'maximal' value_type to the expression tree, which is obtained applying the usual C++ promotion rules and Boost.TypeOf magic. The above is used as follows
template <class Expr>
struct vector_expr : proto::extends <Expr, vector_expr <Expr>, vector_domain>
{
typedef proto::extends <Expr, vector_expr <Expr>, vector_domain> base_type;
// OK! now my expression has a 'value_type'
typedef typename boost::result_of<deduce_value_type(Expr)>::type value_type;
vector_expr (Expr const &e) : base_type (e) {}
};
But now, the following code (check previous question: boost.proto + modify expression tree in place and the code in the accepted answer) is broken (with the usual humongous template instantiation error backtrace, for my pleasure)
int main ()
{
double data[] = {1, 2, 3};
vector<3, double> a(data, data+3), b(data,data+3), c(data,data+3);
auto iter = vector_begin_algo()(a + b);
return 0;
}
The reason is simple. The type of typename boost::result_of<vector_begin_algo(a+b)>::type is:
vector_expr<
basic_expr<
tag::plus
, list2< expr<tag::terminal, term<vector_iterator<double*> >, 0l>
, expr<tag::terminal, term<vector_iterator<double*> >, 0l>
>
,
2l>
>
So, the external vector_expr<...> triggers the evaluation of the nested value_type, but the deduce_value_type algorithm doesn't know how to extract the nested value_type from vector_iterator<double*>. One solution is to define a new traits and modify deduce_value_type as follows
// A further trait
template <class Iter>
struct value_type_trait<vector_iterator<Iter> >
{
typedef typename std::iterator_traits<Iter>::value_type type;
};
// Algorithm to deduce the value type of an expression.
struct deduce_value_type
: proto::or_<
proto::when <vector_terminal, value_type_trait<proto::_value>() >
, proto::when <scalar_terminal, proto::_value>
, proto::when <proto::terminal<vector_iterator<proto::_> > , value_type_trait<proto::_value>()> // <- need this now
, proto::otherwise <
proto::_default<deduce_value_type>()
>
>
{};
There are several problems with this approach, but the most important is: for each typedef or static constant that i find convenient defining in the vector_expr struct, I will need to perform all the above only to have the expression compile, even if an iterator-expression IS-NOT vector-expression and it makes no sense to enlarge the interface of vector_expr to accommodate transformed trees.
The question is: there is a way to transform the vector_expr tree, converting vector nodes into iterator nodes, while at the same time removing the vector-ness from the tree itself so that i do not incur in the above problems?
Thanks in advance, best regards!
UPDATE
Sorry, i changed the last part of the question now that my mind is more clear about what (i think) should be achieved. In the meantime, i tried to solve the thing by myself with a partial success (?), but I feel that there should be a better way (so I still need help!).
It seems to me that the problems come from having all the tree nodes wrapped in the vector_expr thing, that has the side-effect of putting requirement on the terminals (mainly the static stuff for successfully compiling). OTOH, once a valid vector_exp has been constructed (namely: obeying the vector_grammar), then i can transform it to a valid iterator_tree without further checks.
I tried to create a transform that changes back all vector_expr nodes in a tree into 'proto::expr'. The code is as follows:
template <class Expr, long Arity = Expr::proto_arity_c>
struct deep_copy_unwrap_impl;
template <class Expr>
struct deep_copy_unwrap_impl <Expr,0>
{
typedef typename proto::tag_of <Expr>::type Tag;
typedef typename proto::result_of::value<Expr>::type A0;
typedef typename proto::result_of::make_expr<Tag, proto::default_domain, A0>::type result_type;
template<typename Expr2, typename S, typename D>
result_type operator()(Expr2 const &e, S const &, D const &) const
{
return proto::make_expr <Tag, proto::default_domain> (e.proto_base().child0);
}
};
template <class Expr>
struct deep_copy_unwrap_impl <Expr,1>
{
typedef typename proto::tag_of <Expr>::type Tag;
typedef typename proto::result_of::child_c<Expr, 0>::type A0;
typedef typename proto::result_of::make_expr<Tag, proto::default_domain, A0>::type result_type;
template<typename Expr2, typename S, typename D>
result_type operator()(Expr2 const &e, S const &, D const &) const
{
return proto::make_expr <Tag, proto::default_domain> (e.proto_base().child0);
}
};
template <class Expr>
struct deep_copy_unwrap_impl <Expr,2>
{
typedef typename proto::tag_of <Expr>::type Tag;
typedef typename proto::result_of::child_c<Expr, 0>::type A0;
typedef typename proto::result_of::child_c<Expr, 1>::type A1;
typedef typename proto::result_of::make_expr<Tag, proto::default_domain, A0, A1>::type result_type;
template<typename Expr2, typename S, typename D>
result_type operator()(Expr2 const &e, S const &, D const &) const
{
return proto::make_expr <Tag, proto::default_domain> (e.proto_base().child0, e.proto_base().child1);
}
};
struct unwrap : proto::callable
{
template <class Sig> struct result;
template <class This, class Expr>
struct result <This(Expr)>
{
typedef typename
deep_copy_unwrap_impl <Expr>
::result_type type;
};
template <class This, class Expr>
struct result <This(Expr&)>
: result<This(Expr)> {};
template <class This, class Expr>
struct result <This(Expr const&)>
: result<This(Expr)> {};
template <class Expr>
typename result <unwrap(Expr)>::type
operator () (Expr const &e) const
{
return deep_copy_unwrap_impl<Expr>()(e, 0, 0);
}
};
struct retarget
: proto::otherwise <
unwrap(proto::nary_expr<proto::_, proto::vararg<retarget> >)
>
{};
int main ()
{
int data[] = {1, 2, 3};
vector<3, int> a(data, data+3), b(data,data+3), c(data,data+3);
auto x=a+b+c; // <- x is an expression tree made up of vector_expr<...> nodes
auto y=retarget()(x); // <- y is an expression tree made up of proto::expr<...> nodes
return 0;
}
The problem you're running into is that Proto's pass_through transform creates new expressions that are in the same domain as the original. This happens in your vector_begin_algo algorithm, in the otherwise clause. You don't want this, but it's what pass_through gives you. You have two strategies: don't use pass_through, or trick pass_through into building an expression in the default domain.
If you're using the latest version of Proto (1.51), you can use make_expr and an unpacking expression instead of pass_through:
// Turn all vector terminals into vector iterator terminals
struct vector_begin_algo
: proto::or_<
proto::when<
proto::terminal<std::vector<_, _> >
, proto::_make_terminal(
vector_iterator<begin(proto::_value)>(begin(proto::_value))
)
>
, proto::when<
proto::terminal<_>
, proto::_make_terminal(proto::_byval(proto::_value))
>
, proto::otherwise<
proto::lazy<
proto::functional::make_expr<proto::tag_of<_>()>(
vector_begin_algo(proto::pack(_))...
)
>
>
>
{};
proto::lazy is needed here because you first need to build the make_expr function object before you can invoke it. It's not a thing of beauty, but it works.
If you are using an older version of Proto, you can get the same effect by tricking pass_through by first removing the domain-specific wrapper from your expression. First, I write a callable to strip the domain-specific wrapper:
struct get_base_expr
: proto::callable
{
template<typename Expr>
struct result;
template<typename This, typename Expr>
struct result<This(Expr)>
{
typedef
typename boost::remove_reference<Expr>::type::proto_base_expr
type;
};
template<typename Expr>
typename Expr::proto_base_expr operator()(Expr const &expr) const
{
return expr.proto_base();
}
};
Then, the vector_begin_algo would be changed as follows:
// Turn all vector terminals into vector iterator terminals
struct vector_begin_algo
: proto::or_<
proto::when<
proto::terminal<std::vector<_, _> >
, proto::_make_terminal(
vector_iterator<begin(proto::_value)>(begin(proto::_value))
)
>
, proto::when<
proto::terminal<_>
, proto::_make_terminal(proto::_byval(proto::_value))
>
, proto::otherwise<
proto::_byval(proto::pass_through<
proto::nary_expr<_, proto::vararg<vector_begin_algo> >
>(get_base_expr(_)))
>
>
{};
This is also not a work of art, but it gets the job done. Don't forget the proto::_byval to work around the const weirdness in the pass_through transform (which is fixed is boost trunk and will be in 1.52, btw).
I can think of one final solution that takes advantage of the fact that Proto expressions are Fusion sequences of their children. You create a Fusion transform_view that wraps the expression and transforms each child with vector_begin_algo. That gets passed to proto::functional::unpack_expr, much like in the first example with make_expr. You'd need proto::lazy there also, for the same reason.
Thanks for pointing out this limitation on Proto's built-in pass_through transform. It'd be good to have a nicer way to do this.