How Can You Create a Stream from a Constant Memory Buffer Without Altering the Data?

Creating an Input Stream from Constant Memory: Overcoming the Data Alteration Restriction

To address the problem of reading data from a constant memory buffer without altering it, it is necessary to create a custom stream buffer. This can be achieved by defining a class that inherits from the standard std::streambuf and overrides its relevant functions.

Specifically, the membuf class defined below serves as the stream buffer:

struct membuf: std::streambuf {
    membuf(char const* base, size_t size) {
        char* p(const_cast<char*>(base));
        this->setg(p, p, p + size);
    }
};

This class sets up the stream buffer with the provided data buffer and its size, essentially defining the range of data to be read.

To create an input stream based on this buffer, we define the imemstream class, which inherits from both membuf and std::istream:

struct imemstream: virtual membuf, std::istream {
    imemstream(char const* base, size_t size)
        : membuf(base, size)
        , std::istream(static_cast<std::streambuf*>(this)) {
    }
};

This class essentially wraps the custom stream buffer and provides the functionality of an input stream. Now, it's possible to use imemstream as a regular input stream:

imemstream in(data, size);
in >> value;

By utilizing this technique, one can read data from a constant memory buffer as if it were coming from a stream, effectively solving the original problem while maintaining the immutability of the data.

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