Read N bytes from file and append them to a std::vector

I want to rea开发者_运维知识库d N bytes of data from a file stream and append them to a vector. So let's say we have a

basic_ifstream<uint8_t> myFileStream;

and a

vector<uint8_t> myBuffer;

Currently I'm doing something like this:

myBuffer.reserve(N);
for (int i=0; i<N; ++i)
{
    uint8_t tmpByte;
    myFileStream.read(&tmpByte, 1);
    myBuffer.push_back(tmpByte);
}

but this is extremely slow.

Now I tried to let myFileStream.read copy the data directly into the vector. Since a vector stores its elements in a contiguous storage location, I thought that something like this should be possible:

uint8_t* ptr = &myBuffer.back(); // there is already some elements in the buffer (I know)
ptr++; // first element after existing data
myBuffer.resize(myBuffer.size() + N);
myFileStream.read(ptr, N);

But with this I get a runtime error (corruption of heap). What is wrong with this solution? Or is there a better way to do this anyway?

Your problem is that resize may need to reallocate the whole vector, and thus invalidate your previous ptr. You need to take the pointer only after resize.

std::size_t oldSize = myBuffer.size();
// resize first
myBuffer.resize(oldSize + N);
uint8_t* ptr = &myBuffer[oldSize]; // already first element after existing data
myFileStream.read(ptr, N);

Note that as a bonus this implementation will work even if the original vector is empty (for N != 0, of course).