这个答案假设了两件事：
- 您希望二进制 blob 紧密打包（无孔）。
- 您不希望以非对齐方式访问数据成员（与访问按编译器默认方式对齐的数据成员相比，这种方式速度较慢）。
如果是这种情况，那么您应该考虑将大“blob”视为面向字节的流的设计。在此流中，您可以编组/解组填充具有自然对齐的对象的标记/值对。
使用此方案，您可以两全其美。您会得到一个紧凑的 blob，但是一旦从 blob 中提取对象，由于自然对齐，访问对象成员的速度很快。它也是可移植的1 并且不依赖于编译器扩展。缺点是您需要为可以放入 blob 中的每种类型编写样板代码。
基本示例：
#include <cassert>
#include <iomanip>
#include <iostream>
#include <stdint.h>
#include <vector>
enum BlobKey
{
kBlobKey_Widget,
kBlobKey_Gadget
};
class Blob
{
public:
Blob() : cursor_(0) {}
// Extract a value from the blob. The key associated with this value should
// already have been extracted.
template <typename T>
Blob& operator>>(T& value)
{
assert(cursor_ < bytes_.size());
char* dest = reinterpret_cast<char*>(&value);
for (size_t i=0; i<sizeof(T); ++i)
dest[i] = bytes_[cursor_++];
return *this;
}
// Insert a value into the blob
template <typename T>
Blob& operator<<(const T& value)
{
const char* src = reinterpret_cast<const char*>(&value);
for (size_t i=0; i<sizeof(T); ++i)
bytes_.push_back(src[i]);
return *this;
}
// Overloads of << and >> for std::string might be useful
bool atEnd() const {return cursor_ >= bytes_.size();}
void rewind() {cursor_ = 0;}
void clear() {bytes_.clear(); rewind();}
void print() const
{
using namespace std;
for (size_t i=0; i<bytes_.size(); ++i)
cout << setfill('0') << setw(2) << hex << int(bytes_[i]) << " ";
std::cout << "\n" << dec << bytes_.size() << " bytes\n";
}
private:
std::vector<uint8_t> bytes_;
size_t cursor_;
};
class Widget
{
public:
explicit Widget(int a=0, short b=0, char c=0) : a_(a), b_(b), c_(c) {}
void print() const
{
std::cout << "Widget: a_=" << a_ << " b=" << b_
<< " c_=" << c_ << "\n";
}
private:
int a_;
short b_;
long c_;
friend Blob& operator>>(Blob& blob, Widget& widget)
{
// Demarshall members from blob
blob >> widget.a_;
blob >> widget.b_;
blob >> widget.c_;
return blob;
};
friend Blob& operator<<(Blob& blob, Widget& widget)
{
// Marshall members to blob
blob << kBlobKey_Widget;
blob << widget.a_;
blob << widget.b_;
blob << widget.c_;
return blob;
};
};
class Gadget
{
public:
explicit Gadget(long a=0, char b=0, short c=0) : a_(a), b_(b), c_(c) {}
void print() const
{
std::cout << "Gadget: a_=" << a_ << " b=" << b_
<< " c_=" << c_ << "\n";
}
private:
long a_;
int b_;
short c_;
friend Blob& operator>>(Blob& blob, Gadget& gadget)
{
// Demarshall members from blob
blob >> gadget.a_;
blob >> gadget.b_;
blob >> gadget.c_;
return blob;
};
friend Blob& operator<<(Blob& blob, Gadget& gadget)
{
// Marshall members to blob
blob << kBlobKey_Gadget;
blob << gadget.a_;
blob << gadget.b_;
blob << gadget.c_;
return blob;
};
};
int main()
{
Widget w1(1,2,3), w2(4,5,6);
Gadget g1(7,8,9), g2(10,11,12);
// Fill blob with widgets and gadgets
Blob blob;
blob << w1 << g1 << w2 << g2;
blob.print();
// Retrieve widgets and gadgets from blob
BlobKey key;
while (!blob.atEnd())
{
blob >> key;
switch (key)
{
case kBlobKey_Widget:
{
Widget w;
blob >> w;
w.print();
}
break;
case kBlobKey_Gadget:
{
Gadget g;
blob >> g;
g.print();
}
break;
default:
std::cout << "Unknown object type in blob\n";
assert(false);
}
}
}
如果您可以使用 Boost，您可能希望将 Boost.Serialization 与二进制内存流一起使用，例如 answer。
(1) 可移植 意味着源代码可以在任何地方编译。如果传输到具有不同字节序和整数大小的其他机器，生成的二进制 blob 将不可移植。