How to Create an Input Stream from Constant Memory in C ?

Creating an Input Stream from Constant Memory

To read data from a constant memory buffer as if it were a stream, a custom stream buffer can be created. This buffer will reference the constant memory location without modifying its contents.

Implementation:

The following code defines a stream buffer, membuf, and an input stream, imemstream, utilizing the membuf buffer:

#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)) {
    }
};

In this code, membuf is initialized with a constant char pointer and data size. It sets up the input buffer (setg) to point to this data. imemstream inherits from both membuf and std::istream, effectively wrapping the constant memory in an input stream.

Usage:

To use the imemstream, instantiate it with the constant memory pointer and size:

imemstream in(data, size);

Data can then be read from the stream as usual:

in >> x >> y >> w;

Note: The const_cast is necessary because std::streambuf::setg requires a non-const char pointer. While the stream buffer will not modify the data, the interface requires this type for flexibility in "normal" stream buffers.

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