Fencepost error in stream read?

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C++

Hi folks. I need a review of a function I wrote that is suspected to perhaps have a bug in it - off by one is the most likely culprit, but I can't find it.

It is part of a const_buffer_stream class I use to read data off an array. For reasons I cannot use iostream classes.

Basically, it seems to be seeking to the wrong position when reading single bytes out of it. (calling read() with a size of 1. I'm not sure if it happens with larger values.

There could very well be no problem with this code. I just need more eyes to rule it out because as per my previous question (Indexing into a bitmap problem)[^] it was determined that the bug was deeper in my code.

What I have tried:

This function reads a portion of memory out of a const byte array. It's part of a larger const_buffer_stream class. It holds a buffer, a cursor position, and the size.

It returns the number of bytes read (which appears to work)

m_current the current offset into the buffer, as a pointer.
m_size the size of the buffer
m_begin the beginning of the buffer, as a pointer

size_t const_buffer_stream::read(uint8_t* destination,size_t size) {
    if(nullptr==m_current || nullptr==destination || size==0)
        return 0;
    size_t offs=m_current-m_begin;
    if(offs>=m_size)
        return 0;
    if(size+offs>m_size) 
        size = (m_size-offs);
    memcpy(destination,m_current,size);
    m_current+=size;

    return size;
}