c – 使用const成员函数增加type_erasure any

我想在向量中访问类型擦除类型的getter. getter标记为const.不知何故,const-ness不会传播到boost :: any包装器对象.最小的例子

#include <iostream>
#include <vector>

#include <boost/mpl/vector.hpp>
#include <boost/type_erasure/any.hpp>
#include <boost/type_erasure/member.hpp>

using namespace boost;
using namespace boost::type_erasure;

BOOST_TYPE_ERASURE_MEMBER((has_test),test,1)

using AnyTest = any<mpl::vector<copy_constructible<>,has_test<int(double)>,relaxed>>;

struct ATest {
  int test(double x) const {
    return 5;
  }
};

int main() {
  auto xs = std::vector<AnyTest>{};
  xs.push_back(ATest{});

  for (const auto& x : xs) {
    std::cout << x.test(42.0) << '\n';
  }
}

导致错误说

clang++ -O3 -std=c++14    minimal.cc   -o minimal
minimal.cc:28:18: error: member function 'test' not viable: 'this' argument has type 'const
      boost::type_erasure::any<boost::mpl::vector<boost::type_erasure::copy_constructible<boost::type_erasure::_self>,has_test<int (double),boost::type_erasure::_self>,boost::type_erasure::relaxed,mpl_::na,mpl_::na>,boost::type_erasure::_self>',but
      function is not marked const
    std::cout << x.test(42.0) << '\n';
                 ^
minimal.cc:11:39: note: 'test' declared here
BOOST_TYPE_ERASURE_MEMBER((has_test),1)
                                      ^
/opt/local/include/boost/type_erasure/member.hpp:133:9: note: expanded from macro 'BOOST_TYPE_ERASURE_MEMBER'
        member,\
        ^
/opt/local/include/boost/type_erasure/member.hpp:242:64: note: expanded from macro 'BOOST_TYPE_ERASURE_MEMBER_I'
    BOOST_TYPE_ERASURE_MEMBER_II(namespace_name,concept_name,member,N)
                                                               ^
/opt/local/include/boost/type_erasure/member.hpp:170:44: note: expanded from macro 'BOOST_TYPE_ERASURE_MEMBER_II'
        typename rebind_any<Base,R>::type member(                                              \
                                           ^
1 error generated.
make: *** [minimal] Error 1

但是,一旦const auto& for循环中的x更改为auto& x,它有效.这是什么原因以及如何要求包装器对象遵循constness？

解决方法

您需要在概念中指定const-ness,如下所示：

using AnyTest = any<mpl::vector<
                    copy_constructible<>,has_test<int(double),const _self>,relaxed> 
                  >;

在文档page “Basic Usage”上找到

Live On Coliru

// https://stackoverflow.com/questions/32743594/boost-type-erasure-any-with-const-member-function
#include <iostream>
#include <vector>

#include <boost/mpl/vector.hpp>
#include <boost/type_erasure/any.hpp>
#include <boost/type_erasure/member.hpp>

using namespace boost;
using namespace boost::type_erasure;

BOOST_TYPE_ERASURE_MEMBER((has_test),1)

using AnyTest = any<mpl::vector<
                    copy_constructible<>,relaxed> 
                  >;

struct ATest {
    int test(double) const { return 5; }
};

int main() {
    auto xs = std::vector<AnyTest>{};
    xs.push_back(ATest{});

    for (auto const &x : xs) {
        std::cout << x.test(42.0) << '\n';
    }
}

打印

原文链接：https://www.f2er.com/c/118797.html

c – 使用const成员函数增加type_erasure any

解决方法

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