派生自“`boost::asio::streambuf`”

Question

我正在尝试创建我的类，它只是从boost::asio::streambuf派生的公共类，并添加了一些方法：

class my_super_streambuf : public boost::asio::streambuf {
public:  
  void my_super_method();
};

但是，当我简单地将boost::asio::streambuf替换为my_super_streambuf时，会出现错误：

error C2039: 'const_iterator' : is not a member of 'my_super_streambuf'

在……里面

D:\projects\my_super_streambuf\third-party\boost\boost/asio/impl/write.hpp(199) : 
see reference to class template instantiation
'boost::asio::detail::consuming_buffers<boost::asio::const_buffer,
ConstBufferSequence>' being compiled

如何正确地从boost::asio::streambuf中派生

Answer 1

问题不在于如何从boost::asio::streambuf中派生出来。相反，产生的错误是因为编译器选择的是非流write(SyncWriteStream&, const ConstBufferSequence&)重载，而不是write(SyncWriteStream&, basic_streambuf&)。要解决这个问题，可以在调用boost::asio::streambuf时将从write()派生的对象显式转换为boost::asio::streambuf的引用。

class derived_streambuf
  : public boost::asio::streambuf
{};

// ...

derived_streambuf streambuf;
boost::asio::write(socket, static_cast<boost::asio::streambuf&>(streambuf));

要理解这个问题，请考虑有关重载函数的声明：

// ConstBufferSequence
template<
    typename SyncWriteStream,
    typename ConstBufferSequence>
std::size_t write(
    SyncWriteStream&,
    const ConstBufferSequence&);

// Streambuf
template<
    typename SyncWriteStream,
    typename Allocator>
std::size_t write(
    SyncWriteStream&,
    basic_streambuf<Allocator>&);

如果提供derived_streambuf作为第二个参数，则函数的实例化将导致：

// ConstBufferSequence
std::size_t write(..., derived_streambuf&);

// Streambuf
std::size_t write(..., basic_streambuf<char>&);

对于编译器来说，第一个是更好的匹配，因为它是一个精确的匹配，因此它被选中。

下面是编译以下内容的完整示例示范代码：

#include <boost/asio.hpp>

// Derive from boost::asio::streambuf.
class derived_streambuf
  : public boost::asio::streambuf
{};

// Helper function to force type.
template <typename Allocator>
boost::asio::basic_streambuf<Allocator>&
as_streambuf(boost::asio::basic_streambuf<Allocator>& streambuf)
{
  return streambuf;
}

int main()
{
  boost::asio::io_service io_service;
  boost::asio::ip::tcp::socket socket(io_service);
  derived_streambuf streambuf;
  boost::asio::write(socket, static_cast<boost::asio::streambuf&>(streambuf));
  boost::asio::write(socket, as_streambuf(streambuf));
}

Answer 2

您确定"const_iterator“是"boost::asio::streambuf”的公共成员吗？

我试过这些代码，它起作用了：

class MyVector : public std::vector<int>{};
int main(int argc, const char * argv[]) {
    MyVector v;
    v.push_back(1);
    for (MyVector::const_iterator iter = v.begin(); iter != v.end(); iter++) {
        cout << *iter << endl;
    }
}

此外，还包括：

class MyBase {
    public:
        typedef int base_int;
};
class MyDerived : public MyBase {};

MyDerived::base_int P = 5;
MyBase::base_int Q = 5;