I am new to metafunctions. I want to write a function that replaces all matches of a certain type in a compound type with some other type. In example: replace<void *, void, int>::type
should be int *
, replace<void, void, int>::type
should be int
, etc.
I basically failed with two different approaches so far:
template
<
typename C, // Type to be searched
typename X, // "Needle" that is searched for
typename Y // Replacing type
>
struct replace
{
typedef C type;
};
// If the type matches the search exactly, replace
template
<
typename C,
typename Y
>
struct replace<C, C, Y>
{
typedef Y type;
};
// If the type is a pointer, strip it and call recursively
template
<
typename C,
typename X,
typename Y
>
struct replace<C *, X, Y>
{
typedef typename replace<C, X, Y>::type * type;
};
This seemed pretty straight forward to me, but I discovered that when I try replace<void *, void *, int>
, the compiler cannot decide whether to use replace<C, C, Y>
or replace<C *, X, Y>
in that case, so compilation fails.
The next thing I tried is to strip pointers in the base function already:
template
<
typename C,
typename X,
typename Y
>
struct replace
{
typedef typename boost::conditional
<
boost::is_pointer<C>::value,
typename replace
<
typename boost::remove_pointer<C>::type,
X, Y
>::type *,
C
>::type
type;
};
...and this is when I found out that I cannot do that either, because type
is apparently not defined at that point, so I cannot do recursive typedef
from the base function.
Now I am out of ideas. How would you solve such a problem?
Here's a general idea:
template <typename, typename> struct pattern;
template <typename T> struct pattern<T, T>
{
template <typename U> struct rebind
{
typedef U other;
};
};
template <typename A, typename B> struct pattern<A*, B>
{
template <typename U> struct rebind
{
typedef typename pattern<A, B>::template rebind<U>::other * other;
};
};
template <typename Haystack, typename Needle, typename New>
struct replace
{
typedef typename pattern<Haystack, Needle>::template rebind<New>::other type;
};
Test:
#include <demangle.hpp>
#include <iostream>
int main()
{
typedef replace<void, void, int>::type T1;
typedef replace<void*, void, int>::type T2;
std::cout << demangle<T1>() << std::endl;
std::cout << demangle<T2>() << std::endl;
}
Prints:
int
int*
Edit: Here's a somewhat more complete set:
template <typename, typename> struct pattern;
template <typename, typename> struct pattern_aux;
template <typename A, typename B> struct pattern_aux
{
template <typename U> struct rebind
{
typedef typename pattern<A, B>::template rebind<U>::other other;
};
};
template <typename A, typename B, unsigned int N> struct pattern_aux<A[N], B>
{
template <typename U> struct rebind
{
typedef typename pattern<A, B>::template rebind<U>::other other[N];
};
};
template <typename A, typename B> struct pattern
{
template <typename U> struct rebind
{
typedef typename pattern_aux<A, B>::template rebind<U>::other * other;
};
};
template <typename T> struct pattern<T, T>
{
template <typename U> struct rebind
{
typedef U other;
};
};
template <typename A, typename B> struct pattern<A*, B>
{
template <typename U> struct rebind
{
typedef typename pattern<A, B>::template rebind<U>::other * other;
};
};
template <typename A, typename B> struct pattern<A const, B>
{
template <typename U> struct rebind
{
typedef typename pattern_aux<A, B>::template rebind<U>::other const other;
};
};
template <typename A, typename B> struct pattern<A volatile, B>
{
template <typename U> struct rebind
{
typedef typename pattern_aux<A, B>::template rebind<U>::other volatile other;
};
};
template <typename A, typename B> struct pattern<A const volatile, B>
{
template <typename U> struct rebind
{
typedef typename pattern_aux<A, B>::template rebind<U>::other const volatile other;
};
};
template <typename Haystack, typename Needle, typename New>
struct replace
{
typedef typename pattern<Haystack, Needle>::template rebind<New>::other type;
};