Constructing a vector with istream_iterators

问题:

I recall once seeing a clever way of using iterators to read an entire binary file into a vector. It looked something like this:

#include <fstream>
#include <ios>
#include <iostream>
#include <vector>

using namespace std;

int main() {
    ifstream source("myfile.dat", ios::in | ios::binary);
    vector<char> data(istream_iterator(source), ???);
    // do stuff with data
    return 0;
}

The idea is to use vector's iterator range constructor by passing input iterators that specify the entire stream. The problem is I'm not sure what to pass for the end iterator.

How do you create an istream_iterator for the end of a file? Am I completely misremembering this idiom?

回答1:

You want the std::istreambuf_iterator<>, for raw input. The std::istream_iterator<> is for formatted input. As for the end of the file, use the stream iterator's default constructor.

std::ifstream source("myfile.dat", std::ios::binary);
std::vector<char> data((std::istreambuf_iterator<char>(source)),
                       std::istreambuf_iterator<char>());

Edited to satisfy C++'s most vexing parse. Thanks, @UncleBens.

回答2:

In C++11 one could:

std::ifstream source("myfile.dat", std::ios::binary);
std::vector<char> data(std::istreambuf_iterator<char>(source), {});

This shorter form avoids the most vexing parse problem because of the {} argument, which removes ambiguity of it being an argument or a formal parameter.

@wilhelmtell's answer could also be updated to avoid this problem by adopting a brace initializer for data. Still in my view, using {} is more simple and turn the initialization form irrelevant.

EDIT

Or, if we had std::lvalue (and maybe std::xvalue instead of std::move):

#include <vector>
#include <fstream>

template <typename T>
constexpr T &lvalue(T &&r) noexcept { return r; }

int main() {
    using namespace std;

    vector<char> data(
        istreambuf_iterator<char>(lvalue(ifstream("myfile.dat", ios::binary))),
        {}
    );
}