istream vs memory mapping a file?

Question

I am trying to map a file to memory and then parse line by line- is istream what I should be using?

Is istream the same as mapping a file to memory on Windows? I have h

Answer 1

std::istream is an abstract type – you cannot use it directly. You should be deriving from it with a custom array-backed streambuf:

#include 
#include 
#include 
#include 

template>
struct basic_membuf : std::basic_streambuf {
    basic_membuf(CharT const* const buf, std::size_t const size) {
        CharT* const p = const_cast(buf);
        this->setg(p, p, p + size);
    }

    //...
};

template>
struct basic_imemstream
: virtual basic_membuf, std::basic_istream {
    basic_imemstream(CharT const* const buf, std::size_t const size)
    : basic_membuf(buf, size),
      std::basic_istream(static_cast*>(this))
    { }

    //...
};

using imemstream = basic_imemstream;

char const* const mmaped_data = /*...*/;
std::size_t const mmap_size = /*...*/;
imemstream s(mmaped_data, mmap_size);
// s now uses the memory mapped data as its underlying buffer.

As for the memory-mapping itself, I recommend using Boost.Interprocess for this purpose:

#include 
#include 
#include 
#include 

namespace bip = boost::interprocess;

//...

std::string filename = /*...*/;
bip::file_mapping mapping(filename.c_str(), bip::read_only);
bip::mapped_region mapped_rgn(mapping, bip::read_only);
char const* const mmaped_data = static_cast(mapped_rgn.get_address());
std::size_t const mmap_size = mapped_rgn.get_size();

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^{Code for imemstream taken from this answer by Dietmar Kühl.}