Question

I’m reading multiple reports from a HID device into an unsigned char, then trying to copy the data to a std::vector. I’m also writing the data out to a file for hex analysis, whose content appears to be correct when I view it. However, the std::vector doesn’t appear to contain the correct data when I dump it to the console.

This is the code:

typedef vector<unsigned char> buffer_t;

buffer_t sendCommand (hid_device *devh, const unsigned char cmd[], int reports) {
    unsigned char outbuf[0x40];
    buffer_t retbuf(0x40 * reports);

    hid_write(devh, cmd, 0x41);

    int i;
    FILE *file = fopen("test.out", "w+b");
    while (i++ < reports) {
       hid_read(devh, outbuf, 0x40);
       fwrite(outbuf, 1, sizeof(outbuf), file);
       retbuf.push_back(*outbuf);
    }
    fclose(file);
    cout << &retbuf[0];
    return retbuf;
}

I have a feeling I’m way off the mark here; I’m fairly new to C/C++, and I’ve been stuck with this for a while now. Can anyone tell me what I’m doing wrong, or point me in a better direction?

Answer 1

You want to add multiple unsigned char objects to your vector, but push_back only adds one.

So, replace retbuf.push_back(*outbuf); with either:

for (size_t i = 0; i < sizeof(outbuf); ++i) {
    retbuf.push_back(outbuf[i]);
}

or

std::copy(outbuf, outbuf+sizeof(outbuf), std::back_inserter(retbuf));

or

retbuf.insert(retbuf.end(), outbuf, outbuf+sizeof(outbuf));

which all do the same thing.

You create your vector with a certain size:

buffer_t retbuf(0x40 * reports);

but push_back increases the size of the vector by adding an element at the end. You should create it empty:

buffer_t retbuf;

Optionally, you could arrange for the vector to have enough space allocated, ready for the elements you’re going to add:

retbuf.reserve(0x40 * reports);

This is purely a performance issue, but sometimes it’s a significant issue for large vectors, or vectors of types that (unlike unsigned char) are expensive to copy/move when the vector runs out of internal space and has to allocate more.

A note on style: you repeat the literal value 0x40 a few times, and also use sizeof(outbuf). It’s often best to define a constant, and use the name throughout:

const int report_size = 0x40;

This is partly in case the number changes in future, but also it’s about the readability of your code — if someone sees 0x40 they may or may not immediately understand why that is the correct value. If someone sees report_size then they don’t know what value that actually is until they look it up, but they do know why you’re using that value.