Reading n chars from stream to string

Question

I need to read n chars from a binary file into a string. Currently, what I do is:

static string Read(istream &stream, uint32_t count)
{
    auto bytes = unique_ptr<char[]>(new char[count]);

    stream.read(bytes.get(), count);

    return string(bytes.get(), count);
}

I found the way I deal with the array of chars quite messy. If I used new and delete[] directly, it would make the code messy in another way (I would need to add a local variable for the result). And I'm trying to avoid delete as a general rule.

Is there a clear way to write this code? The fact that it uses twice as much memory as it needs is probably not a big deal, but fixing that would be nice too.

Answer 1

Why not:

static string Read(istream &stream, uint32_t count)
{
    std::string result(count, ' ');
    stream.read(&result[0], count);

    return result;
}

Though not strictly C++03 compatible that is easily validated. One of the reasons the committee found it easy to add the new constraint in C++11 was that no implementation did not use contiguous memory (Random Access Iterators are a hint).

But a C++03 strict implementation would be:

static string Read(istream &stream, uint32_t count)
{
    std::vector<char> result(count);  // Because vector is guranteed to be contiguous in C++03
    stream.read(&result[0], count);

    return std::string(&result[0], &result[count]);
}

Answer 2

First of all, it seems that you've got a using namespace std; somewhere in your code. Don't do that. (No, really).

Here's a function that should meet your needs.

static std::string Read(std::istream &stream, std::string::size_type count)
{
    std::string out;
    out.reserve(count);
    std::copy_n(std::istreambuf_iterator(stream), count, std::back_inserter(out));
    return out;
}