I guess this is what you're looking for - a stream buffer that returns a pointer to its buffer:
#include <iostream>
#include <vector>
#include <string>
class raw_buffer : public std::streambuf
{
public:
raw_buffer(std::ostream& os, int buf_size = 256);
int_type overflow(int_type c) override;
std::streamsize showmanyc() override;
std::streamsize xsputn(const char_type*, std::streamsize) override;
int sync() override;
bool flush();
std::string const& str() const;
private:
std::ostream& os_;
std::vector<char> buffer;
std::string aux;
};
Now str()
is simple. It returns a pointer to the underlying buffer of the auxillary buffer:
std::string const& raw_buffer::str() const
{
return aux;
}
The rest of the functions are the usual implementations for a stream buffer. showmanyc()
should return the size of the auxiliary buffer (aux
is just a running total of the entire buffer, buffer
on the other hand is the size specified at construction).
For example, here is overflow()
, which should update both buffers at same time but still treat buffer
as the primary buffer:
raw_buffer::int_type raw_buffer::overflow(raw_buffer::int_type c) override
{
if (os_ && !traits_type::eq_int_type(c, traits_type::eof()))
{
aux += *this->pptr() = traits_type::to_char_type(c);
this->pbump(1);
if (flush())
{
this->pbump(-(this->pptr() - this->pbase()));
this->setp(this->buffer.data(),
this->buffer.data() + this->buffer.size());
return c;
}
}
return traits_type::eof();
}
flush()
is used to copy the contents of buffer
to the stream (os_
), and sync()
should be overrided to call flush()
too.
xsputn
also needs to be overrided to write to aux
as well:
std::streamsize raw_buffer::xsputn(const raw_buffer::char_type* str, std::streamsize count) override
{
for (int i = 0; i < count; ++i)
{
if (traits_type::eq_int_type(this->sputc(str[i]), traits_type::eof()))
return i;
else
aux += str[i];
}
return count;
}
Now we can put this together with a customized stream:
class raw_ostream : private virtual raw_buffer
, public std::ostream
{
public:
raw_ostream(std::ostream& os) : raw_buffer(os)
, std::ostream(this)
{ }
std::string const& str() const
{
return this->raw_buffer::str();
}
std::streamsize count()
{
return this->str().size();
}
};
It can be used like this:
int main()
{
raw_ostream rostr(std::cout);
rostr << "Hello, World " << 123 << true << false;
auto& buf = rostr.str();
std::cout << buf;
}