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Eggs.TypePatterns

Introduction

Eggs.TypePatterns is pattern matching for types. It uses a terse syntax to check for multiple type traits, and extract components of compound types.

The library is loosely based on Boost.Proto pattern matching for proto expression grammars. It makes extensive use of the C++ Standard Library as well as the Boost C++ Libraries.

The library is in its early development stages, and currently provides no examples, tests nor documentation. Its use in a production environment is not recommended at the time.

Motivation

Given a type T, we need to check whether it is a pointer-to-const U and if so we have to find the type U.

Using the type traits components from the C++11 standard library, we could do:

using namespace std;

bool matches =
  is_pointer< T >::value && 
  is_const< typename remove_pointer< T >::type >::value;

typedef typename remove_const<
  typename remove_pointer< T >::type
>::type U;

With the addition of transformation traits redux to C++14, we can get ride of those typenames:

using namespace std;

bool matches =
  is_pointer< T >::value &&
  is_const< remove_pointer_t< T > >::value;

typedef remove_const_t<
  remove_pointer_t< T >
> U;

Using TypePatterns, we get a more expressive solution:

using namespace eggs::type_patterns;

bool matches = match< T, _1 const* >::value;

typedef typename at< match< T, _1 const* >, _1 >::type U;

Basic usage

Main components

Use the match template metafunction to match a type against a given pattern. Use placeholders to build more complex patterns. [ Example:

#include <eggs/type_patterns.hpp>
using namespace eggs::type_patterns;

template< typename T >
struct is_member_pointer
  : match< T, _1 _2::* > //~> Pattern: _1 _2::*
{
    typedef typename at<
            is_member_pointer, _
        >::type type; //~> type matched, T
    typedef typename at<
            is_member_pointer, _1
        >::type member_type; //~> type that matched _1
    typedef typename at<
            is_member_pointer, _2
        >::type class_type; //~> type that matched _2
};

struct X { int foo; };

static_assert(
    is_member_pointer< decltype(&X::foo) >::value
  , "oops! &X::foo should be a member pointer" );
static_assert(
    !is_member_pointer< decltype(X::foo) >::value
  , "oops! X::foo should be an int" );

—end example ]

Placeholders

Placeholders allow you to build more powerful patterns.

  • _ is a wildcard placeholder, it matches any type.

  • _1 .. _9 are capture placeholders, they match any type the first time they are encountered. Afterwards, they only match the type that was initially matched.

  • N is a size placeholder, usable to match arrays of any size.

  • varargs is a variadic placeholder, it matches zero or more types, usable to match functions or templates with any number of trailing arguments.

Metafunctions

Metafunctions allow you to extend the matching functionality beyond the syntax of C++ types.

  • apply< Predicate > matches a type when evaluating the predicate for said type results in a true type. In this context, _ resolves to the type being matched and _1 .. _9 resolve to already matched capture placeholders.

  • as< Placeholder, Pattern > performs a pattern match against the type, and it captures it in the given capture placeholder when it does match.

  • not_< Pattern > matches a type that does not match the given pattern.

  • ignore_const< Pattern >, ignore_volatile< Pattern >, ignore_cv< Pattern > perform a pattern match that ignores const and/or volatile qualifiers.

  • ignore_signed< Pattern >, ignore_unsigned< Pattern >, ignore_sign< Pattern > perform a pattern match that ignores signed and/or unsigned modifiers.

  • all_of< ...Patterns > matches a type that matches all of the patterns.

  • any_of< ...Patterns > matches a type that matches any of the patterns.

  • none_of< ...Patterns > matches a type that matches none of the patterns.

Copyright Agustín Bergé, Fusion Fenix 2013

Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)