CODEWITHSUNDEEP
c++vectorbinary-data
ahMarrone
ahMarrone

Reputation: 191

C++ binary file - Writing ints - strange behaviour

I have a simple vector of ints, and I want to write it into a binary file. For example:

#include <fstream>
#include <vector>

int main () {
    std::vector<uint32_t> myVector{5, 10, 15, 20 };
    // write vector to bin file
    std::ofstream outfile("./binary_ints.data",  std::ios_base::binary|std::ios::trunc);
    std::copy(myVector.begin(), myVector.end(), std::ostreambuf_iterator<char>(outfile));
    outfile.close(); 
    return 0;
}

Then, if I inspect the file "binary_ints.data" in hex mode, I have this:

00000000: 050a 0f14 0a

Thats ok!.

However, If myVector has this data:

std::vector<uint32_t> myVector{3231748228};

Then, the hex stored is weird:

00000000: 840a

84 in Hex doesn't match with the Int 3231748228.

What's happening here? Thanks.

Upvotes: 0

Views: 152

Answers (1)

Innokentiy Alaytsev
Innokentiy Alaytsev

Reputation: 840

The problem is that each value in your std::vector<uint32_t> is interpreted as char during your std::copy () invocation. 3231748228 is represented in hex as ‭C0A09084. std::copy () takes uint32_t value, truncates it to single byte which is 0x84 on Little-endian processor. After writing byte 0x84 in file byte 0x0a is added which corresponds to new line character.

A possible solution is to use ofstream::write() instead of std::copy ():

#include <fstream>
#include <vector>

int main () {
    std::vector<uint32_t> myVector{3231748228 };
    // write vector to bin file
    std::ofstream outfile("./binary_ints.data",  std::ios_base::binary|std::ios::trunc);

    outfile.write (
        (char*)(myVector.data ()), 
        myVector.size () * sizeof (decltype (myVector)::value_type));

    outfile.close();
    return 0;
}

Note the use of decltype (). The same effect may be achivied by just writing sizeof (uint32_t), but with decltype () you may be sure that the code remains correct even if you change myVector value type.

Upvotes: 3

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