copy member variable into byte vector - c++

Question

I want to copy a 64-bit member variable into a vector byte by byte.
Please avoid telling me to use bit operation to extract each byte and then copy them into vector.
I want to do this by one line.
I use memcpy and copy methods, but both of them failed.
Here is the sample code:
#include <iostream>
#include <vector>
#include <cstdint>
#include <cstring>
using namespace std;
class A {
public:
A()
: eight_bytes_data(0x1234567812345678) {
}
void Test() {
vector<uint8_t> data_out;
data_out.reserve(8);
memcpy(data_out.data(),
&eight_bytes_data,
8);
cerr << "[Test]" << data_out.size() << endl;
}
void Test2() {
vector<uint8_t> data_out;
data_out.reserve(8);
copy(&eight_bytes_data,
(&eight_bytes_data) + 8,
back_inserter(data_out));
cerr << "[Test2]" << data_out.size() << endl;
for (auto value : data_out) {
cerr << hex << value << endl;
}
}
private:
uint64_t eight_bytes_data;
};
int main() {
A a;
a.Test();
a.Test2();
return 0;
}

Answer 1

As the others already showed where you were getting wrong, there is a one line solution that is dangeurous.
First you need to make sure that you vector has enough size to receive 8 bytes. Something like this:
data_out.resize(8);
The you can do a reinterpret_cast to force your compiler to interpret those 8 bytes from the vector to be seen as an unique type of 8 bytes, and do the copy
*(reinterpret_cast<uint64_t*>(data_out.data())) = eight_bytes_data;
I can't figure out all the possibilities of something going wrong. So use at your own risk.

Answer 2

If you want to work with the bytes of another type structure, you could use a char* to manipulate each byte:
void Test3()
{
vector<uint8_t> data_out;
char* pbyte = (char*)&eight_bytes_data;
for(int i = 0; i < sizeof(eight_bytes_data); ++i)
{
data_out.push_back(pbyte[i]);
}
cerr << "[Test]" << data_out.size() << endl;
}
Unfortunately, you requested a one-line-solution, which I don't think is viable.

Answer 3

If you are interested in more generic version:
namespace detail
{
template<typename Byte, typename T>
struct Impl
{
static std::vector<Byte> impl(const T& data)
{
std::vector<Byte> bytes;
bytes.resize(sizeof(T)/sizeof(Byte));
*(T*)bytes.data() = data;
return bytes;
}
};
template<typename T>
struct Impl<bool, T>
{
static std::vector<bool> impl(const T& data)
{
std::bitset<sizeof(T)*8> bits(data);
std::string string = bits.to_string();
std::vector<bool> vector;
for(const auto& x : string)
vector.push_back(x - '0');
return vector;
}
};
}
template<typename Byte = uint8_t,
typename T>
std::vector<Byte> data_to_bytes(const T& data)
{
return detail::Impl<Byte,T>::impl(data);
}
int main()
{
uint64_t test = 0x1111222233334444ull;
for(auto x : data_to_bytes<bool>(test))
std::cout << std::hex << uintmax_t(x) << " ";
std::cout << std::endl << std::endl;
for(auto x : data_to_bytes(test))
std::cout << std::hex << uintmax_t(x) << " ";
std::cout << std::endl << std::endl;
for(auto x : data_to_bytes<uint16_t>(test))
std::cout << std::hex << uintmax_t(x) << " ";
std::cout << std::endl << std::endl;
for(auto x : data_to_bytes<uint32_t>(test))
std::cout << std::hex << uintmax_t(x) << " ";
std::cout << std::endl << std::endl;
for(auto x : data_to_bytes<uint64_t>(test))
std::cout << std::hex << uintmax_t(x) << " ";
std::cout << std::endl << std::endl;
}
Output:
0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 1 0 0 1 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 1
0 0 0 1 0 0 0 1 0 0 0 1 0 0
44 44 33 33 22 22 11 11
4444 3333 2222 1111
33334444 11112222
1111222233334444