Question

如何获取范围内Boost Phoenix语句中使用的局部变量类型？使用Phoenix和Proto我可以提取Phoenix表达的许多方面。例如，以下代码公开了arity（3）;标签类型（lambda_actor）;和lambda lambda表达式的child-2标签类型（shift_left）：

#include <boost/proto/proto.hpp>
#include <boost/phoenix.hpp>

namespace proto   = boost::proto;
namespace phoenix = boost::phoenix;
using namespace phoenix::local_names;

struct Foo { const char str[6] = " Ok.\n"; };

int main(int argc, char *argv[])
{
  auto f = phoenix::lambda(_a = 17, _b = Foo()) [
    std::cout << _a << phoenix::bind(&Foo::str,_b)
  ];

  typedef typename proto::tag_of<decltype(                  f )>::type tag;
  typedef typename proto::tag_of<decltype(proto::child_c<2>(f))>::type tagc;
  static_assert(proto::arity_of<decltype(f)>::value==3,"");
  static_assert(std::is_same<tag,  phoenix::tag::lambda_actor>::value,"");
  static_assert(std::is_same<tagc, proto::tag::shift_left>::value,"");

  return 0;
}

如何获取局部变量的类型;在此示例中：_a和_b？

Answer 1

我假设你感兴趣的类型是int和Foo，如果这不是你想要的，请忽略这个答案。查看文档，我无法找到获得这些类型的简单方法。但是，如果你查看f中存储的proto表达式的类型，你会发现int和Foo可以在第一个孩子内的actor的向量中找到。您可以在输出中看到最终到达有趣类型所需的步骤，然后您可以轻松创建一个可以满足您需要的元函数。在这个简单的情况下，get_local_type使用索引来访问相关类型。如果要通过名称访问它（使用_a），您应该能够使用lambda表达式的第二个子项中map_local_index_to_tuple中的数据来获取与名称关联的索引。使用phoenix::detail::get_index定义here实施get_local_type_from_name也非常简单。这个元函数需要上面提到的映射作为它的第一个参数和你想要信息的占位符的类型（更具体地说它需要phoenix::detail::local<phoenix::local_names::_a_key>，你可以使用proto::result_of::value来获得占位符的类型。作为第二个。

#include <iostream>
#include <typeinfo>
#include <string>
#include <cxxabi.h>
#include <type_traits>

#include <boost/proto/proto.hpp>
#include <boost/phoenix.hpp>

namespace proto   = boost::proto;
namespace phoenix = boost::phoenix;
using namespace phoenix::local_names;

namespace fusion = boost::fusion;

struct Foo { const char str[6] = " Ok.\n"; };



std::string demangle(const char* mangledName) {
    int status;
    char* result = abi::__cxa_demangle(mangledName, nullptr, nullptr, &status);
    switch(status) {
    case -1:
        std::cerr << "Out of memory!" << std::endl;
        exit(1);
    case -2:
        return mangledName;
    case -3: // Should never happen, but just in case?
        return mangledName;
    }
    std::string name = result;
    free(result);
    return name;
}


template <typename Lambda, int N>
struct get_local_type
{
    typedef typename proto::result_of::value<typename proto::result_of::child_c<Lambda,0>::type >::type vector_of_locals_type;
    typedef typename proto::result_of::value<typename fusion::result_of::at_c<vector_of_locals_type,N>::type >::type ref_type;
    typedef typename std::remove_reference<ref_type>::type type;
};

template <typename Lambda, typename Arg>
struct get_local_type_from_name
{
    typedef typename proto::result_of::value<Arg>::type local_name;
    typedef typename proto::result_of::value<typename proto::result_of::child_c<Lambda,1>::type >::type map_type;
    typedef typename phoenix::detail::get_index<map_type,local_name>::type index;
    typedef typename get_local_type<Lambda,index::value>::type type;

};



int main(int argc, char *argv[])
{
  auto f = phoenix::lambda(_b = 17, _a = Foo()) [
    std::cout << _b << phoenix::bind(&Foo::str,_a)
  ];

  std::cout << std::endl << "This is the whole lambda expression:" << std::endl;
  std::cout << std::endl << demangle(typeid(f).name()) << std::endl;
  std::cout << std::endl << "Take the first child:" << std::endl;
  std::cout << std::endl << demangle(typeid(proto::child_c<0>(f)).name()) << std::endl;
  std::cout << std::endl << "Then its value (this is a vector that contains the types you want):" << std::endl;
  std::cout << std::endl << demangle(typeid(proto::value(proto::child_c<0>(f))).name()) << std::endl;
  std::cout << std::endl << "Take the first element of that vector:" << std::endl;
  std::cout << std::endl << demangle(typeid(fusion::at_c<0>(proto::value(proto::child_c<0>(f)))).name()) << std::endl;
  std::cout << std::endl << "Take the value of that element:" << std::endl;
  std::cout << std::endl << demangle(typeid(proto::value(fusion::at_c<0>(proto::value(proto::child_c<0>(f))))).name()) << std::endl;


  typedef typename proto::tag_of<decltype(                  f )>::type tag;
  typedef typename proto::tag_of<decltype(proto::child_c<2>(f))>::type tagc;
  static_assert(proto::arity_of<decltype(f)>::value==3,"");
  static_assert(std::is_same<tag,  phoenix::tag::lambda_actor>::value,"");
  static_assert(std::is_same<tagc, proto::tag::shift_left>::value,"");

  typedef typename get_local_type<decltype(f),0>::type type_of_1st;
  typedef typename get_local_type<decltype(f),1>::type type_of_2nd;
  typedef typename get_local_type_from_name<decltype(f),_a_type>::type type_of_a;
  typedef typename get_local_type_from_name<decltype(f),decltype(_b)>::type type_of_b;
  static_assert(std::is_same<type_of_1st,int>::value,"");
  static_assert(std::is_same<type_of_2nd,Foo>::value,"");
  static_assert(std::is_same<type_of_a,Foo>::value,"");
  static_assert(std::is_same<type_of_b,int>::value,"");


  return 0;
}

在Boost Phoenix中获取局部变量的类型

1 个答案: