简而言之，我想知道boost :: serialization在通过指针反序列化时如何为对象分配内存。在下面，您将找到我的问题的示例，在伴随代码的旁边清楚地说明了该问题。该代码应具有完整的功能并可以正确编译，本身没有错误，只是有关代码实际工作方式的问题。
#include <cstddef> // NULL
#include <iomanip>
#include <iostream>
#include <fstream>
#include <string>
#include <boost/archive/text_iarchive.hpp>
#include <boost/archive/text_oarchive.hpp>
class non_default_constructor; // Forward declaration for boost serialization namespacing below
// In order to "teach" boost how to save and load your class with a non-default-constructor, you must override these functions
// in the boost::serialization namespace. Prototype them here.
namespace boost { namespace serialization {
template<class Archive>
inline void save_construct_data(Archive& ar, const non_default_constructor* ndc, const unsigned int version);
template<class Archive>
inline void load_construct_data(Archive& ar, non_default_constructor* ndc, const unsigned int version);
}}
// Here is the actual class definition with no default constructor
class non_default_constructor
{
public:
explicit non_default_constructor(std::string initial)
: some_initial_value{initial}, state{0}
{
}
std::string get_initial_value() const { return some_initial_value; } // For save_construct_data
private:
std::string some_initial_value;
int state;
// Notice that we only serialize state here, not the
// some_initial_value passed into the ctor
friend class boost::serialization::access;
template<class Archive>
void serialize(Archive& ar, const unsigned int version)
{
std::cout << "serialize called" << std::endl;
ar & state;
}
};
// Define the save and load overides here.
namespace boost { namespace serialization {
template<class Archive>
inline void save_construct_data(Archive& ar, const non_default_constructor* ndc, const unsigned int version)
{
std::cout << "save_construct_data called." << std::endl;
ar << ndc->get_initial_value();
}
template<class Archive>
inline void load_construct_data(Archive& ar, non_default_constructor* ndc, const unsigned int version)
{
std::cout << "load_construct_data called." << std::endl;
std::string some_initial_value;
ar >> some_initial_value;
// Use placement new to construct a non_default_constructor class at the address of ndc
::new(ndc)non_default_constructor(some_initial_value);
}
}}
int main(int argc, char *argv[])
{
// Now lets say that we want to save and load a non_default_constructor class through a pointer.
non_default_constructor* my_non_default_constructor = new non_default_constructor{"initial value"};
std::ofstream outputStream("non_default_constructor.dat");
boost::archive::text_oarchive outputArchive(outputStream);
outputArchive << my_non_default_constructor;
outputStream.close();
// The above is all fine and dandy. We've serialized an object through a pointer.
// non_default_constructor will call save_construct_data then will call serialize()
// The output archive file will look exactly like this:
/*
22 serialization::archive 17 0 1 0
0 13 initial value 0
*/
/*If I want to load that class back into an object at a later time
I'd declare a pointer to a non_default_constructor */
non_default_constructor* load_from_archive;
// Notice load_from_archive was not initialized with any value. It doesn't make
// sense to intialize it with a value, because we're trying to load from
// a file, not create a whole new object with "new".
std::ifstream inputStream("non_default_constructor.dat");
boost::archive::text_iarchive inputArchive(inputStream);
// <><><> HERE IS WHERE I'M CONFUSED <><><>
inputArchive >> load_from_archive;
// The above should call load_construct_data which will attempt to
// construct a non_default_constructor object at the address of
// load_from_archive, but HOW DOES IT KNOW HOW MUCH MEMORY A NON_DEFAULT_CONSTRUCTOR
// class uses?? Placement new just constructs at the address, assuming
// memory at the passed address has been allocated for construction.
// So my question is this:
// I want to verify that *something* is (or isn't) allocating memory for a non_default_constructor
// class to be constructed at the address of load_from_archive.
std::cout << load_from_archive->get_initial_value() << std::endl; // This works.
return 0;
}
[boost::serialization documentation when a class with a non-default constructor要进行反序列化，将使用load / save_construct_data，但实际上我没有看到要为要装入的对象分配内存的位置，只是正在构造new位置内存地址中的对象。但是是什么分配了该地址的内存？
对这条线的工作方式可能有误解：
::new(ndc)non_default_constructor(some_initial_value);
但是我想知道我的误会在哪里。这是我的第一个问题，如果对我的提问方式有误，我深表歉意。谢谢您的时间。
这是一个出色的示例程序，带有非常恰当的注释。让我们深入。
// In order to "teach" boost how to save and load your class with a
// non-default-constructor, you must override these functions in the
// boost::serialization namespace. Prototype them here.
您不必。通过ADL可以访问的任何重载（非覆盖）都足够，除了in-class选项。
跳过它的肉：
// So my question is this: I want to verify that *something* is (or isn't)
// allocating memory for a non_default_constructor
// class to be constructed at the address of load_from_archive.
是。文档指出了这一点。但这有点棘手，因为它是有条件的。原因是对象跟踪。说，我们序列化指向同一对象的多个指针，它们将被序列化一次。
反序列化时，对象将在归档流中使用对象tracking-id表示。仅第一个实例将导致分配。
请参见documentation。
这是一个简化的反例：
它使用指针的10个副本序列化向量。我使用unique_ptr避免泄漏实例（既是在main中手动创建的实例，也是通过反序列化创建的实例）。
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打印
#include <iomanip>
#include <iostream>
#include <fstream>
#include <boost/archive/text_iarchive.hpp>
#include <boost/archive/text_oarchive.hpp>
#include <boost/serialization/vector.hpp>
namespace mylib {
// Here is the actual class definition with no default constructor
class non_default_constructor {
public:
explicit non_default_constructor(std::string initial)
: some_initial_value{ initial }, state{ 0 } {}
std::string get_initial_value() const {
return some_initial_value;
} // For save_construct_data
private:
std::string some_initial_value;
int state;
// Notice that we only serialize state here, not the some_initial_value
// passed into the ctor
friend class boost::serialization::access;
template <class Archive> void serialize(Archive& ar, unsigned) {
std::cout << "serialize called" << std::endl;
ar& state;
}
};
// Define the save and load overides here.
template<class Archive>
inline void save_construct_data(Archive& ar, const non_default_constructor* ndc, unsigned)
{
std::cout << "save_construct_data called." << std::endl;
ar << ndc->get_initial_value();
}
template<class Archive>
inline void load_construct_data(Archive& ar, non_default_constructor* ndc, unsigned)
{
std::cout << "load_construct_data called." << std::endl;
std::string some_initial_value;
ar >> some_initial_value;
// Use placement new to construct a non_default_constructor class at the address of ndc
::new(ndc)non_default_constructor(some_initial_value);
}
}
int main() {
using NDC = mylib::non_default_constructor;
auto owned = std::make_unique<NDC>("initial value");
{
std::ofstream outputStream("vector.dat");
boost::archive::text_oarchive outputArchive(outputStream);
// serialize 10 copues, for fun
std::vector v(10, owned.get());
outputArchive << v;
}
/*
22 serialization::archive 17 0 0 10 0 1 1 0
0 13 initial value 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0
*/
std::vector<NDC*> restore;
{
std::ifstream inputStream("vector.dat");
boost::archive::text_iarchive inputArchive(inputStream);
inputArchive >> restore;
}
std::unique_ptr<NDC> take_ownership(restore.front());
for (auto& el : restore) {
assert(el == take_ownership.get());
}
std::cout << "restored: " << restore.size() << " copies with " <<
std::quoted(take_ownership->get_initial_value()) << "\n";
}
save_construct_data called.
serialize called
load_construct_data called.
serialize called
restored: 10 copies with "initial value"
文件包含：
vector.dat您不应该在乎，但是您当然可以阅读源代码。可以预料的是，它毕竟比您天真地期望的要复杂得多：这是C ++。
该库处理已重载22 serialization::archive 17 0 0 10 0 1 1 0
0 13 initial value 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0
的类型。在这种情况下，它将调用operator new而不是全局变量T::operator new。只要您正确推测，它就会始终通过operator new。
代码位于异常安全包装器中：sizeof(T)
detail/iserializer.hpp
是的，此代码在C ++ 11或更高版本中已大大简化。同样，析构函数中的NULL防护对于struct heap_allocation {
explicit heap_allocation() { m_p = invoke_new(); }
~heap_allocation() {
if (0 != m_p)
invoke_delete(m_p);
}
T* get() const { return m_p; }
T* release() {
T* p = m_p;
m_p = 0;
return p;
}
private:
T* m_p;
};
的兼容实现是多余的。
现在当然是operator delete和invoke_new。简明呈现：
invoke_delete有一些条件编译和冗长的注释，因此，如果需要完整的图片，请使用源代码。