How do I combine multiple char reads into a std::vector? - c++

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.

Answer 2

The problem is in this line: buffer_t retbuf(0x40 * reports); It means that you create vector with 0x40 * reports elements filled with default value for unsigned char (zero). Then push_back() just adds new elements to the end of vector and doesn't affect existing elements.
You need to rewrite it this way:
buffer_t retbuf; // Empty vector
retbuf.reserve(0x40 * reports); // Preallocate memory for known element count
This way push_back() will work as expected and add elements to empty vector from beginning.
And of course you shall push_back() all elements of outbuf, not only first one (*outbuf).

Answer 3

To push back multiple values use std::vector's function assign. For example:
std::vector<char>vec1;
char array[3] = {'a', 'b', 'c'};
vec1.assign(array, array+3);
I am currently working on a project were I had to do this.

Answer 4

Your vector is of a type unsigned char, which means every element of it is of this type. Your outbuf is an array of unsigned chars.
The push_back() only appends one item to the end of the vector, so push_back(*outbuf) will only add the first element of the outbuf to the vector, not all of them.
To put all the data into the vector, you will need to push_back them one-by-one, or use std::copy.

Answer 5

Note that since outbuf is a char array, then *outbuf will be the first element of the char array because of the array/pointer duality.
I think you probably wanted to do:
typedef vector<string> buffer_t; // alternatively vector<unsigned char*>
...
retbuf.push_back(outbuf);
...
Or
typedef vector<unsigned char> buffer_t;
...
for (size_t i = 0; i < sizeof(outbuf); i++)
retbuf.push_back(outbuf);
...