CODEWITHSUNDEEP
c++vectorstlstreamifstream
Robinson
Robinson

Reputation: 10122

How can I read an entire stream of bytes into an std::vector?

I read an answer here showing how to read an entire stream into a std::string with the following one (two) liner:

std::istreambuf_iterator<char> eos;    
std::string s(std::istreambuf_iterator<char>(stream), eos);

For doing something similar to read a binary stream into a std::vector, why can't I simply replace char with uint8_t and std::string with std::vector?

auto stream = std::ifstream(path, std::ios::in | std::ios::binary);    
auto eos = std::istreambuf_iterator<uint8_t>();
auto buffer = std::vector<uint8_t>(std::istreambuf_iterator<uint8_t>(stream), eos);

The above produces a compiler error (VC2013):

1>d:\non-svn\c++\library\i\file\filereader.cpp(62): error C2440: '' : cannot convert from 'std::basic_ifstream>' to 'std::istreambuf_iterator>' 1>
with 1> [ 1> _Elem=uint8_t 1> ] 1>
No constructor could take the source type, or constructor overload resolution was ambiguous

Upvotes: 8

Views: 13970

Answers (3)

einpoklum
einpoklum

Reputation: 131415

I'd say the problem is with what you're trying to do:

For doing something similar to read a binary stream into an std::vector

Don't use iteration for reading bytes into a buffer, my friend!

Instead, do something like this:

std::ifstream file(path, std::ios::binary);
auto file_contents = [&file]() {
    file.seekg(0, std::ios::end);
    auto file_size = file.tellg();
    file.seekg(0, std::ios::beg);
    std::vector<byte_type> file_contents(file_size);
    file.read(result.data(), file_size);
    return file_contents;
}();

or actually, since you want to handle errors, this:

std::ifstream file(path, std::ios::binary);
auto file_contents = [&file]() {
    try {
        file.exceptions(std::ios::failbit | std::ios::badbit);
        file.seekg(0, std::ios::end);
        auto file_size = file.tellg();
        file.seekg(0, std::ios::beg);
        std::vector<byte_type> file_contents(file_size);
        file.read(result.data(), file_size);
        return file_contents;
    } catch (std::ios_base::failure& ios_failure) {
        if (errno == 0) { throw ios_failure; }
        throw std::system_error(errno, std::generic_category(), "read from binary file");
    }
}();

Upvotes: 0

Barry
Barry

Reputation: 302663

There's just a type mismatch. ifstream is just a typedef:

typedef basic_ifstream<char> ifstream;

So if you want to use a different underlying type, you just have to tell it:

std::basic_ifstream<uint8_t> stream(path, std::ios::in | std::ios::binary);    
auto eos = std::istreambuf_iterator<uint8_t>();
auto buffer = std::vector<uint8_t>(std::istreambuf_iterator<uint8_t>(stream), eos);

That works for me.

Or, since Dietmar says this might be a little sketchy, you could do something like:

auto stream = std::ifstream(...);
std::vector<uint8_t> data;

std::for_each(std::istreambuf_iterator<char>(stream),
              std::istreambuf_iterator<char>(),
              [&data](const char c){
                  data.push_back(c);
              });

Upvotes: 17

Mike Seymour
Mike Seymour

Reputation: 254431

ifstream is a stream of char, not uint8_t. You'll need either basic_ifstream<uint8_t> or istreambuf_iterator<char> for the types to match.

The former may not work without some amount of work, since the library is only required to support streams of char and wchar_t; so you probably want istreambuf_iterator<char>.

Upvotes: 6

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