I have some data in a buffer pointed to by a const char*
pointer. The data is just an ASCII string. I know its size. I would like to be able to read it in the same way data is read from streams. I'm looking for a solution that would allow me to write code like this:
// for example, data points to a string "42 3.14 blah"
MemoryStreamWrapper in(data, data_size);
int x;
float y;
std::string w;
in >> x >> y >> w;
Important condition: the data must not be copied or altered in any way (otherwise I'd just use a string stream. To my best knowledge, it isn't possible to create a string stream from a const char pointer without copying the data.)
The way to do this is to create a suitable stream buffer. This can, e.g., be done like this:
#include <streambuf>
#include <istream>
struct membuf: std::streambuf {
membuf(char const* base, size_t size) {
char* p(const_cast<char*>(base));
this->setg(p, p, p + size);
}
};
struct imemstream: virtual membuf, std::istream {
imemstream(char const* base, size_t size)
: membuf(base, size)
, std::istream(static_cast<std::streambuf*>(this)) {
}
};
The only somewhat awkward thing is the const_cast<char*>()
in the stream buffer: the stream buffer won't change the data but the interface still requires char*
to be used, mainly to make it easier to change the buffer in "normal" stream buffers. With this, you can use imemstream
as a normal input stream:
imemstream in(data, size);
in >> value;