How do I read an entire file into a std::string in C++?

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One way is to flush the stream buffer into a separate memory stream, and then convert that to std::string (error handling omitted):

std::string slurp(std::ifstream& in) {
    std::ostringstream sstr;
    sstr << in.rdbuf();
    return sstr.str();
}

This is nicely concise. However, as noted in the question this performs a redundant copy and unfortunately there is fundamentally no way of eliding this copy.

The only real solution that avoids redundant copies is to do the reading manually in a loop, unfortunately. Since C++ now has guaranteed contiguous strings, one could write the following (≥C++17, error handling included):

auto read_file(std::string_view path) -> std::string {
    constexpr auto read_size = std::size_t(4096);
    auto stream = std::ifstream(path.data());
    stream.exceptions(std::ios_base::badbit);
    
    auto out = std::string();
    auto buf = std::string(read_size, '\0');
    while (stream.read(& buf[0], read_size)) {
        out.append(buf, 0, stream.gcount());
    }
    out.append(buf, 0, stream.gcount());
    return out;
}

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