How to read from RAM?

Question

What I need to do?

I need to read the file from RAM

How I am doing it?

I use std::istream, because it know how to read from buffer. So, there is method that know how to read file size(according to this SO answer https://stackoverflow.com/a/6039648/5709159)

long getFileSize(const std::string &filename)
{
    struct stat stat_buf{};
    int rc = stat(filename.c_str(), &stat_buf);
    return rc == 0 ? stat_buf.st_size : -1;
}

Then I use membuf to copy bytes from file to buffer (according to this SO answer https://stackoverflow.com/a/13586356/5709159)

struct membuf : std::streambuf
    {
        membuf(char *base, std::ptrdiff_t n)
        {
            this->setg(base, base, base + n);
        }
    };

Full implementation looks like this

long size = getFileSize(filename);
membuf sbuf(0, size);
std::istream file(&sbuf);

But, I am now sure that sbuf(0, size); that 0 here is on the right place...

So, question is - how to set membuf with right size ?

Answer 1

Option 1

Expanding on the code in your question:

long size = getFileSize(filename);
std::vector<char> buffer(size);
// here fill the buffer with data
membuf sbuf(buffer.data(), buffer.size());
std::istream file_from_ram(&sbuf);

if you'd like to fill the buffer with contenst of some file, do this:

std::istream original_file("filename.dat");
original_file.read(buffer.data(), size);

Note: this method has problems. First, as @andrew-henle mentioned in comments, file size of type off_t which may not fil into long. Second, this method suffers from TOCTOU (time of check -- time of use) problem: if file size changes between the call to getFileSize and reading the file, you may be in trouble.

Option 2

Use std::istringstream instead:

std::string buffer;
// here fill the string with data
std::istringstream file_from_ram(buffer);

If you'd like to fill the buffer with contents of some file, do this (from https://stackoverflow.com/a/116220/4451432):

std::ifstream original_file("filename.dat");

std::istringstream file_from_ram;
file_from_ram << original_file.rdbuf();
// Now you can read from file_from_ram