那些遇到过的问题

boost::serialization：具有私有默认构造函数的对象在向量中工作，但在映射中不起作用

Phi*_*ipp 4 c++ boost stdmap boost-serialization

考虑以下代码：

#include <boost/serialization/nvp.hpp>
#include <boost/archive/xml_iarchive.hpp>
#include <boost/archive/xml_oarchive.hpp>

class Foo{
    friend class boost::serialization::access;
    template<class Archive>
    void serialize(Archive & ar, const unsigned int)
    {
        ar & BOOST_SERIALIZATION_NVP(i);
    }
    int i;
    Foo():i(0){}
public:
    Foo(int k):i(k){}
};

int main(int argc, char *argv[])
{
    std::vector< Foo> f;
    f.push_back(Foo(12));
    std::ofstream os("path");
    boost::archive::xml_oarchive oa(os);
    oa << boost::serialization::make_nvp("f", f);
    os.close();
    std::vector<Foo> g;
    std::ifstream is("path");
    boost::archive::xml_iarchive ia(is);
    ia >> boost::serialization::make_nvp("f", g);
}
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在序列化 Foos 向量时效果很好。但是，如果我尝试序列化 Foos 的映射，它会在私有默认构造函数上失败：

std::map<std::string, Foo> f;
f.insert(std::make_pair("hello", Foo(12)));
std::ofstream os("path");
boost::archive::xml_oarchive oa(os);
oa << boost::serialization::make_nvp("f", f);
os.close();
std::map<std::string, Foo> g;
std::ifstream is("path");
boost::archive::xml_iarchive ia(is);
ia >> boost::serialization::make_nvp("f", g);
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失败了

In file included from main.cpp:2:
In file included from /usr/local/include/boost/serialization/nvp.hpp:19:
In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/utility:70:
/usr/bin/../lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/bits/stl_pair.h:109:18: error: field of type 'Foo' has private default constructor
      : first(), second() { }
                 ^
/usr/local/include/boost/serialization/access.hpp:132:17: note: in instantiation of member function 'std::pair<const std::basic_string<char>, Foo>::pair' requested here
        ::new(t)T;
                ^
/usr/local/include/boost/serialization/serialization.hpp:93:13: note: in instantiation of function template specialization 'boost::serialization::access::construct<std::pair<const std::basic_string<char>, Foo> >' requested here
    access::construct(t);
            ^
/usr/local/include/boost/serialization/serialization.hpp:158:9: note: in instantiation of function template specialization 'boost::serialization::load_construct_data<boost::archive::xml_iarchive, std::pair<const std::basic_string<char>, Foo> >' requested here
        load_construct_data(ar, t, v);
        ^
/usr/local/include/boost/serialization/detail/stack_constructor.hpp:58:31: note: in instantiation of function template specialization 'boost::serialization::load_construct_data_adl<boost::archive::xml_iarchive, std::pair<const std::basic_string<char>, Foo> >' requested here
        boost::serialization::load_construct_data_adl(
                              ^
/usr/local/include/boost/serialization/collections_load_imp.hpp:83:48: note: in instantiation of member function 'boost::serialization::detail::stack_construct<boost::archive::xml_iarchive, std::pair<const std::basic_string<char>, Foo> >::stack_construct' requested here
        detail::stack_construct<Archive, type> t(ar, v);
                                               ^
/usr/local/include/boost/serialization/collections_load_imp.hpp:158:16: note: (skipping 12 contexts in backtrace; use -ftemplate-backtrace-limit=0 to see all)
        hint = ifunc(ar, s, item_version, hint);
               ^
/usr/local/include/boost/archive/detail/common_iarchive.hpp:66:18: note: in instantiation of function template specialization 'boost::archive::load<boost::archive::xml_iarchive, std::map<std::basic_string<char>, Foo, std::less<std::basic_string<char> >, std::allocator<std::pair<const std::basic_string<char>, Foo> > > >' requested here
        archive::load(* this->This(), t);
                 ^
/usr/local/include/boost/archive/basic_xml_iarchive.hpp:86:39: note: in instantiation of function template specialization 'boost::archive::detail::common_iarchive<boost::archive::xml_iarchive>::load_override<std::map<std::basic_string<char>, Foo, std::less<std::basic_string<char> >, std::allocator<std::pair<const std::basic_string<char>, Foo> > > >' requested here
        this->detail_common_iarchive::load_override(t.value(), 0);
                                      ^
/usr/local/include/boost/archive/xml_iarchive.hpp:93:38: note: in instantiation of function template specialization 'boost::archive::basic_xml_iarchive<boost::archive::xml_iarchive>::load_override<std::map<std::basic_string<char>, Foo, std::less<std::basic_string<char> >, std::allocator<std::pair<const std::basic_string<char>, Foo> > > >' requested here
        basic_xml_iarchive<Archive>::load_override(t, 0);
                                     ^
/usr/local/include/boost/archive/detail/interface_iarchive.hpp:60:23: note: in instantiation of function template specialization 'boost::archive::xml_iarchive_impl<boost::archive::xml_iarchive>::load_override<const boost::serialization::nvp<std::map<std::basic_string<char>, Foo, std::less<std::basic_string<char> >, std::allocator<std::pair<const std::basic_string<char>, Foo> > > > >' requested here
        this->This()->load_override(t, 0);
                      ^
main.cpp:50:8: note: in instantiation of function template specialization 'boost::archive::detail::interface_iarchive<boost::archive::xml_iarchive>::operator>><const boost::serialization::nvp<std::map<std::basic_string<char>, Foo, std::less<std::basic_string<char> >, std::allocator<std::pair<const std::basic_string<char>, Foo> > > > >' requested here
    ia >> boost::serialization::make_nvp("f", g);
       ^
main.cpp:34:5: note: implicitly declared private here
    Foo():i(0){}
    ^
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我正在使用 clang Ubuntu clang 版本 3.4-1ubuntu3 (tags/RELEASE_34/final) (基于 LLVM 3.4)

以及 Ubuntu 14.04LTS 附带的 boost 版本 1.55。

我尝试提供 load_construct_data() 函数，如下所示：

namespace boost
{
    namespace serialization
    {
        template<class Archive>
        inline void load_construct_data(Archive &archive, Foo*a, unsigned int
file_version)
        {
            ::new(a)Foo(0);
        }
    }
}
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但我仍然遇到相同的错误，因为它在实例化 std::pair 时需要构造函数

seh*_*ehe 5

哦。啊哈。

\n\n

我刚刚使用 Boost 1.57.0 来比较情况map<string, Foo>。

\n\n

嗯，你很幸运。您发现了另一个库版本依赖项（可能是一个错误）。

\n\n
    \n

  • 不使用它，而是提供私有默认构造函数，GCC 4.8.2 可以很好地编译它：Live On Coliru [1]

    • \n

  • GCC 4.9.0 无法编译它（它也使用了更新版本的标准库）。默认构造函数std::pair<>无法在那里编译，因为Foo不可默认构造：Live On Coliru

    • \n
\n\n

解决方案

\n\n

save_construct_data幸运的是，使用/的解决方案 load_construct_data再次挽救了局面。

\n\n

但是，您需要考虑以下事实：元素类型实际上不是Foo，而是std::pair<T const, Foo>。

\n\n
template <class Archive, typename K> inline friend void save_construct_data(Archive& ar, std::pair<K, Foo> const* v, const unsigned int) {\n    std::cerr << __PRETTY_FUNCTION__ << "\\n";\n    ar & boost::serialization::make_nvp("first", v->first);\n    ar & boost::serialization::make_nvp("second", v->second.i);\n}\ntemplate <class Archive, typename K> inline friend void load_construct_data(Archive& ar, std::pair<K, Foo>* v, const unsigned int) {\n    std::cerr << __PRETTY_FUNCTION__ << "\\n";\n    typename std::remove_cv<K>::type first;\n    ar & boost::serialization::make_nvp("first", first);\n    int tmp;\n    ar & boost::serialization::make_nvp("second", tmp);\n    new(v) std::pair<K, Foo>(first, tmp);\n}\n
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现在一切正常了：

\n\n

Live On Coliru

\n\n
#include <boost/serialization/nvp.hpp>\n#include <boost/serialization/serialization.hpp>\n#include <boost/archive/xml_iarchive.hpp>\n#include <boost/archive/xml_oarchive.hpp>\n#include <boost/serialization/access.hpp>\n#include <boost/serialization/vector.hpp>\n#include <boost/serialization/map.hpp>\n#include <boost/version.hpp>\n#include <fstream>\n#include <iostream>\n\nclass Foo {\n    friend class boost::serialization::access;\n\n    template <class Archive> void serialize(Archive &, const unsigned int) {\n        std::cerr << __PRETTY_FUNCTION__ << "\\n";\n    }\n\n    template <class Archive, typename K> inline friend void save_construct_data(Archive& ar, std::pair<K, Foo> const* v, const unsigned int) {\n        std::cerr << __PRETTY_FUNCTION__ << "\\n";\n        ar & boost::serialization::make_nvp("first", v->first);\n        ar & boost::serialization::make_nvp("second", v->second.i);\n    }\n    template <class Archive, typename K> inline friend void load_construct_data(Archive& ar, std::pair<K, Foo>* v, const unsigned int) {\n        std::cerr << __PRETTY_FUNCTION__ << "\\n";\n        typename std::remove_cv<K>::type first;\n        ar & boost::serialization::make_nvp("first", first);\n        int tmp;\n        ar & boost::serialization::make_nvp("second", tmp);\n        new(v) std::pair<K, Foo>(first, tmp);\n    }\n\n    int i;\n\n  public:\n    Foo(int k) : i(k) {}\n\n    friend std::ostream& operator<<(std::ostream& os, Foo const& foo) {\n        return os << "Foo { " << foo.i << " }";\n    }\n};\n\nnamespace boost { namespace serialization {\n\n\n} }\n\nint main() {\n    using Data = std::map<std::string, Foo>;\n    std::cout << "Boost version: " << BOOST_VERSION << "\\n";\n\n    {\n        auto f = Data { {"a", 12 }, {"b", 42} };\n        //for (auto& e : f) std::cout << e.first << ", " << e.second << "\\n";\n        std::ofstream os("path");\n        boost::archive::xml_oarchive oa(os);\n        oa << boost::serialization::make_nvp("f", f);\n    }\n\n    {\n        Data g;\n        std::ifstream is("path");\n        boost::archive::xml_iarchive ia(is);\n        ia >> boost::serialization::make_nvp("f", g);\n\n        for (auto& e : g)\n            std::cout << e.first << ", " << e.second << "\\n";\n    }\n}\n
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哪个打印：

\n\n
Boost version: 105700\nvoid save_construct_data(Archive&, const std::pair<K, Foo>*, unsigned int) [with Archive = boost::archive::xml_oarchive; K = const std::__cxx11::basic_string<char>]\nvoid Foo::serialize(Archive&, unsigned int) [with Archive = boost::archive::xml_oarchive]\nvoid save_construct_data(Archive&, const std::pair<K, Foo>*, unsigned int) [with Archive = boost::archive::xml_oarchive; K = const std::__cxx11::basic_string<char>]\nvoid Foo::serialize(Archive&, unsigned int) [with Archive = boost::archive::xml_oarchive]\nvoid load_construct_data(Archive&, std::pair<K, Foo>*, unsigned int) [with Archive = boost::archive::xml_iarchive; K = const std::__cxx11::basic_string<char>]\nvoid Foo::serialize(Archive&, unsigned int) [with Archive = boost::archive::xml_iarchive]\nvoid load_construct_data(Archive&, std::pair<K, Foo>*, unsigned int) [with Archive = boost::archive::xml_iarchive; K = const std::__cxx11::basic_string<char>]\nvoid Foo::serialize(Archive&, unsigned int) [with Archive = boost::archive::xml_iarchive]\na, Foo { 12 }\nb, Foo { 42 }\n
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\n\n

[1]（我无法在 Coliru 上链接它，因为那里的 boost 库已被重新编译为 GCC 5.0 ABI）

\n\n

笔记

\n\n

更好的通用解决方案是load/save_construct_data对命名空间中的非默认可构造类型进行通用处理boost::serialization。这样，人们就不必“了解”std::pair<>实施细节。他们可以只load/save_construct_data为自己的用户类型实现，无论他们将其放入向量还是地图中，都将 JustWork\xe2\x84\xa2 。

\n\n

不过，实现这一点通常并不简单，并且可能会干扰 Boost Serialization 框架内部的一些其他机制。

\n\n

我更愿意得到 Boost Serialization 维护人员的帮助，以可靠的方式做到这一点。所以，看来我今天要提交两张票。

\n

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