Moving a class derived from istream

问题

I'm creating a C++ istream with a custom streambuf. Trying to move this fails because the istream move constructor is protected. To get round this I derived a class from istream:

struct VectorCharBuf : public streambuf {
    VectorCharBuf(vector<char>& v) {
        setg(v.data(), v.data(), v.data() + v.size());
    }
};

struct IVectorCharStream : public istream {
    IVectorCharStream(VectorCharBuf* contents_buf) : istream(contents_buf) {}
};

The default move constructor for this class is not generated because it involves the

use of deleted function 'std::basic_ios<_CharT, _Traits>::basic_ios(const std::basic_ios<_CharT, _Traits>&) [with _CharT = char; _Traits = std::char_traits]'

Further, if I try to declare a move constructor explicitly, as in

struct MyIStream : public istream {
    MyIStream(MyIStream&& str) : istream(move(str)) {}
};

I get an error "invalid use of void expression". (I'm probably doing something silly in this last case, but I just can't spot it... .)

How can I create a movable istream?

回答1:

struct MyIStream : public istream {
    MyIStream(MyIStream&& str) : istream(move(str)) {}
};

I get an error "invalid use of void expression".

This doesn't work because basic_ios, a base class of istream contains a member function void move(basic_ios& _Other) (for moving the base class).

If you use std::move(str) in the constructor, it compiles!

回答2:

How can I create a movable istream?

You can implement your class in terms of your base class' default constructor, instead of the move constructor of the base:

struct MyIStream : public istream {
    MyIStream(MyIStream&& str)
   : istream()
   {
       // todo: implement move functionality from str
   }
};

来源：https://stackoverflow.com/questions/39394523/moving-a-class-derived-from-istream

标签

c++

c++11

move

istream