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在另一个命名空间中定义符号

namespaces templates c++

3

Luc Touraille · 技术社区 · 16 年前

type_to_string

namespace foo {

    template <typename T>
    struct type_to_string
    {
        static const char * value;
    };
}

template <typename T>
const char * foo::type_to_string<T>::value = "???";

#define CREATE_ID(name)                               \
struct name;                                          \
                                                      \
template<>                                            \
const char * foo::type_to_string<name>::value = #name;

namespace bar
{
    CREATE_ID(baz)
}

type_to_string<T>::value 必须在封闭的命名空间中定义 foo .

[COMEAU 4.3.10.1] error: member "foo::type_to_string<T>::value [with T=bar::baz]"
cannot be specialized in the current scope

[VISUAL C++ 2008] error C2888: 'const char *foo::type_to_string<T>::value'Â :
symbol cannot be defined within namespace 'bar'
     with
     [
         T=bar::baz
     ]

在宏中,每个名称空间都有自己的版本,或者类似的东西)?

2 回复 | 直到 16 年前

1

9

Kirill V. Lyadvinsky 16 年前

根据C++标准14.7.3/2:

#define DECL_ID(name) \
struct name;                                          

#define CREATE_ID(name) \
template<>              \
const char * foo::type_to_string<name>::value = #name;

namespace bar { namespace bar2 {
    DECL_ID(baz)
} }
CREATE_ID(bar::bar2::baz)

或

#define CREATE_ID(ns, name)     \
namespace ns { struct name; }   \
                                \
template<>                      \
const char * foo::type_to_string<ns::name>::value = #name;

CREATE_ID(bar, baz)

value

#define DECL_ID(name) \
struct name;                                          

#define CREATE_ID(ns, name) \
template<>              \
const char * foo::type_to_string<ns::name>::value = #name;

namespace bar { namespace bar2 {
    DECL_ID(baz)
} }
CREATE_ID(bar::bar2, baz)

2

1

Luc Touraille 16 年前

#include <boost/preprocessor/seq/for_each.hpp>
#include <boost/preprocessor/seq/size.hpp>
#include <boost/preprocessor/repetition/repeat.hpp>

#define BEGIN_NS(r, _, elem)  namespace elem {
#define CLOSE_NS(z, n, _)     }
#define APPEND_NS(r, _, elem) elem::

#define CREATE_ID(ns_list, name)                         \
                                                         \
BOOST_PP_SEQ_FOR_EACH(BEGIN_NS, ~, ns_list)              \
    struct name;                                         \
BOOST_PP_REPEAT(BOOST_PP_SEQ_SIZE(ns_list), CLOSE_NS, ~) \
                                                         \
template<>                                               \
const char * devs::type_to_string<                       \
    BOOST_PP_SEQ_FOR_EACH(APPEND_NS, ~, ns_list)name     \
>::value = #name;

CREATE_ID((bar) (bar2), baz)