Using a callback to consume data from std::vector<uint8_t> - c++

Question

Based on the previous answers, I wrote a better buffer that can be consumed fast:
#include <iostream>
#include <vector>
#include <functional>
#include <stdio.h>
#include <string.h>
using namespace std;
template <class T>
class Buffer
{
public:
Buffer(size_t reserve) {
_buffer.reserve(reserve);
}
void write(T* buffer, size_t amountToWrite) {
this->_buffer.insert(this->_buffer.begin(), buffer, buffer + amountToWrite);
}
size_t consume(const size_t amountToConsume, std::function<void(T*, T*)> onConsume) {
if (_current==this->_buffer.size()) {
//this means we've already read everything. Maybe we should throw or just return 0?
return 0;
}
size_t currentActualSize = this->_buffer.size() - this->_current;
if (amountToConsume <= currentActualSize) {
onConsume(this->_buffer.data() + this->_current, this->_buffer.data() + this->_current + amountToConsume);
this->_current += amountToConsume;
return amountToConsume;
} else {
size_t remaining = currentActualSize;
onConsume(this->_buffer.data() + this->_current, this->_buffer.data() + this->_current + remaining);
this->_current += remaining;
return remaining;
}
}
private:
std::vector<T> _buffer;
size_t _current = 0;
};
int main()
{
Buffer<uint8_t> b(7);
uint8_t u[] = {1,2,3,4,5,6,7};
b.write(u, 7);
uint8_t r[3] = {255, 255, 255};
size_t consumedBytes2 = b.consume(3, [r](const uint8_t* begin, const uint8_t* end){
std::cout << "begin: " << begin << ", end: " << end << std::endl;
std::copy(begin, end, (uint8_t*)r);
});
std::cout << "consumed " << consumedBytes2 << std::endl;
for(int i=0; i<consumedBytes2; i++) {
std::cout << (int) r[i] << " - ";
}
std::cout << std::endl;
return 0;
}
and I added a callback that calls with the buffer begin and end, so I can write to whenever I want, instead of copying to a temporary buffer and then writing to another buffer.
However, as you can see, the output is:
begin: , end:
consumed 3
255 - 255 - 255 -
nothing is being written to my buffer.
What is happening?

Answer 1

You are capturing your array by value.
Capturing by reference will give you the expected result.
size_t consumedBytes2 = b.consume(3, [&r](const uint8_t* begin, const uint8_t* end){
std::cout << "begin: " << begin << ", end: " << end << std::endl;
std::copy(begin, end, (uint8_t*)r);
});
Capturing a pointer by value would have worked fine, but the array doesn't decay to a pointer here as it would when passed to a function.