Read a file backwards?

Use a memory-mapped file and walk backwards. The OS will page in the needed parts of the file in reverse order.

As per comment, a possible (quite simple) alternative would be read the lines into a vector. For example:

#include <iostream>
#include <fstream>
#include <string>
#include <vector>

int main()
{
    std::ifstream in("main.cpp");

    if (in.is_open())
    {
        std::vector<std::string> lines_in_reverse;
        std::string line;
        while (std::getline(in, line))
        {
            // Store the lines in reverse order.
            lines_in_reverse.insert(lines_in_reverse.begin(), line);
        }
    }
}

EDIT:

As per jrok's and Loki Astari's comments, push_back() would be more efficient but the lines would be in file order, so reverse iteration (reverse_iterator) or std::reverse() would be necessary:

    std::vector<std::string> lines_in_order;
    std::string line;
    while (std::getline(in, line))
    {
        lines_in_order.push_back(line);
    }