c++boostmetaprogrammingtemplate-meta-programmingboost-mpl

Ambiguous metafunction or undefined type


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?


Solution

  • 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;
    };