reading selected lines of file using mmap - c++

Question

I posted one question, which was related to faster reading of a file, by skipping specific lines but that does not seem to go well with standard c++ api's.
I researched more and got to know what memory mapped files could come handy for these kinds of cases. Details about memory mapped files are here.
All in all,
Suppose, the file(file.txt) is like this:
A quick brown fox
// Blah blah
// Blah blah
jumps over the little lazy dog
And then in code, opened file, Read that as memory mapped file and then iterate over the contents of the char* pointer, skipping the file pointers itself. Wanted to give it a try before reaching to an conclusion on it. Skeleton of my code looks like this:
struct stat filestat;
FILE *file = fopen("file.txt", "r");
if (-1 == fstat(fileno(file), &filestat)) {
std::cout << "FAILED with fstat" << std::endl;
return FALSE;
} else {
char* data = (char*)mmap(0, filestat.st_size, PROT_READ, MAP_PRIVATE, fileno(file), 0);
if (data == 0) {
std::cout << "FAILED " << std::endl;
return FALSE;
}
// Filter out 'data'
// for (unsigned int i = 0; i < filestat.st_size; ++i) {
// Do something here..
// }
munmap(data, filestat.st_size);
return TRUE;
}
In this case, I would want to capture lines which does not start with //. Since this file(file.txt) is already memory mapped, I could go over the data pointer and filter out the lines. Am I correct in doing so?
If so, what is the efficient way to parse the lines?

Answer 1

Reading selected lines from wherever and copy them to whatever can be done with the C++ algorithms.
You can use std::copy_if. This will copy data from any source to any destination, if the predicate is true.
I show you a simple example that copies data from a file and skips all lines starting with "//". The result will be put in a vector.
This is one statement with calling one function. So, a classical one liner.
For debugging purposes, I print the result to the console.
#include <iostream>
#include <vector>
#include <iterator>
#include <algorithm>
#include <string>
#include <fstream>
using LineBasedTextFile = std::vector<std::string>;
class CompleteLine { // Proxy for the input Iterator
public:
// Overload extractor. Read a complete line
friend std::istream& operator>>(std::istream& is, CompleteLine& cl) { std::getline(is, cl.completeLine); return is; }
// Cast the type 'CompleteLine' to std::string
operator std::string() const { return completeLine; }
protected:
// Temporary to hold the read string
std::string completeLine{};
};
int main()
{
// Open the input file
std::ifstream inputFile("r:\\input.txt");
if (inputFile)
{
// This vector will hold all lines of the file
LineBasedTextFile lineBasedTextFile{};
// Read the file and copy all lines that fullfill the required condition, into the vector of lines
std::copy_if(std::istream_iterator<CompleteLine>(inputFile), std::istream_iterator<CompleteLine>(), std::back_inserter(lineBasedTextFile), [](const std::string & s) {return s.find("//") != 0; });
// Print vector of lines
std::copy(lineBasedTextFile.begin(), lineBasedTextFile.end(), std::ostream_iterator<std::string>(std::cout, "\n"));
}
return 0;
}
I hope this helps