I have a tuple type. I want to add a element type in it to get a new tuple type. I can do it like
decltype tuple_cat(MyTuple, std::tuple<MyType>())
However, I don't find tuple_cat
in boost::tuple
, how to do it in boost?
Creating a Tuple A tuple in Python can be created by enclosing all the comma-separated elements inside the parenthesis (). Elements of the tuple are immutable and ordered. It allows duplicate values and can have any number of elements.
A tuple can have any number of items and they may be of different types (integer, float, list, string, etc.). A tuple can also be created without using parentheses.
I assume you want all this in compile time.
Here is the general explanation: concatening tuples is similar to concatening lists or arrays, is that the algorithm is the same. Here, given tuples a
and b
, I choosed to move the last element of a
to the beginning of b
, and repeat until a
is empty.
First: base structures. The following structure keeps a parameter pack. It can be anything, for example a tuple:
template<typename... T>
struct pack
{
static const unsigned int size = sizeof...(T);
};
Note that the size of the pack is stored inside it. It is not mandatory, but it is convenient for the explanation. Boost uses the struct boost::tuples::length<T>::value
(which is more verbose).
To access an element at i-th position, we use a structure similar to boost::tuples::element<n, T>
:
// Get i-th element of parameter pack
// AKA 'implementation'
// Principle: the element i is the first element of the sub-array starting at indice i-1
template<int n, typename F, typename... T>
struct element_at : public element_at<n-1, T...>
{
};
template<typename F, typename... T>
struct element_at<0, F, T...>
{
typedef F type;
};
// Get i-th element of pack
// AKA 'interface' for the 'pack' structure
template<int n, typename P>
struct element
{
};
template<int n, typename... T>
struct element<n, pack<T...>>
{
typedef typename element_at<n, T...>::type type;
};
Now, we must use a low-level operation which is adding one element to a side of a pack (adding at left or at right). Here adding at left is choosed, but it is not the only choice:
// Concat at left (only for structure 'pack')
template<typename a, typename b>
struct tuple_concat_left
{
};
template<typename a, typename... b>
struct tuple_concat_left<a, pack<b...>>
{
typedef pack<a, b...> type;
};
For templates, a
is not changed, and instead we use an indice to know what element to add. The inheritance define a 'type' typedef which is the concatenation of all indices after n
and the other tuple (not including n
, and in order). We just have to concatenate at left the element at indice n
.
// Concat 2 tuples
template<typename a, typename b, int n = 0, bool ok = (n < a::size)>
struct tuple_concat : public tuple_concat<a, b, n+1>
{
typedef typename tuple_concat_left<
typename element<n, a>::type,
typename tuple_concat<a, b, n+1>::type
>::type type;
};
template<typename a, typename b, int n>
struct tuple_concat<a, b, n, false>
{
typedef b type;
};
And that's it! Live example here.
Now, for tuple specifics: you noticed I didn't used boost::tuple nor std::tuple. That is because a lot of implementations of boost tuplesdo not have access to variadic templates, so a fixed number of template parameters is used (they default to boost::tuples::null_type
). Putting this directly with variadic templates is a headache, thus the need to have another abstraction.
I also assumed that you can use C++11 (with the decltype
in your question). Concatening 2 tuples in C++03 is possible, but more repetitive and boring.
You can convert a pack
to a tuple really easily: just change the pack
definition to:
template<typename... T>
struct pack
{
static const unsigned int size = sizeof...(T);
typedef boost::tuple<T...> to_tuple; // < convert this pack to a boost::tuple
};
C++14 offers a library to generate a sequence of integers at compile type. This helps manipulating static sequences like tuples and arrays (example). An integer sequence can be obtained
template<size_t... Ints>
struct integer_sequence {};
template<size_t Size, size_t... Ints>
struct implementation : implementation<Size-1, Size-1, Ints...> {};
template<size_t... Ints>
struct implementation<0, Ints...>
{
typedef integer_sequence<Ints...> type;
};
template<class... T>
using index_sequence_for = typename implementation<sizeof...(T)>::type;
To concat MyTuple
and MyType
you can write the simple functions:
template<typename X, typename Tuple, size_t... Ints>
auto concat(X x, Tuple t, integer_sequence<Ints...>)
-> decltype( std::make_tuple(x, std::get<Ints>(t)...) )
{
return std::make_tuple(x, std::get<Ints>(t)...);
}
template<typename X, typename... T>
std::tuple<X, T...> concat(X x, std::tuple<T...> t)
{
return concat(x, t, index_sequence_for<T...>());
}
concat(MyType, MyTuple);
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