Converting std::vector<char> to char* causes defective characters - c++

Question

I have a function in my code called buildPacket that takes some parameters, and converts them into a char* and adds them together using a std::vector<char> and at the end returns the result as a char*. The problem is that after I convert the vector to a char* all characters become a weird character.
I tried using other ways of converting the vector to a char*, like with using reinterpret_cast<char*>. When I print the contents of the vector from inside the function, I get the expected result so the problem is with the conversion.
The function's code:
char* buildPacket (int code, std::string data)
{
char* codeBytes = CAST_TO_BYTES(code);
std::vector<char> packetBytes(codeBytes, codeBytes + sizeof(char));
size_t dataLength = data.size() + 1;
char* dataLengthBytes = CAST_TO_BYTES(dataLength);
packetBytes.insert(packetBytes.end(), dataLengthBytes, dataLengthBytes + sizeof(int));
const char* dataBytes = data.c_str();
packetBytes.insert(packetBytes.end(), dataBytes, dataBytes + dataLength);
return &packetBytes[0];
}
The CAST_TO_BYTES macro:
#define CAST_TO_BYTES(OBJ) static_cast<char*>(static_cast<void*>(&OBJ));
The intent of the function is to take the input and build a packet out of it to send through a socket later on, the packet's format consists of a 1-byte long code, 4-byte long data length and data with variable length.
The input I gave it is code = 101 and data = "{\"password\":\"123456\",\"username\":\"test\"}"
This is the result I am getting when printing the characters: ▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌▌
EDIT: Thanks for all the help, I've returned a vector<char> at the end as suggested and took a different approach in converting to values to a char*.

Answer 1

You're returning a pointer to something inside of a local variable. You should change your code to have your vector<char> alive outside of your buildPacket function (such as by returning it instead of the char*).

Answer 2

You might try this solution. I thing using STL makes it more clearer what you are trying to achieve. There was also an undefined reference in your code, that could lead to unpredictable crashes.
#include <iostream>
#include <string>
#include <vector>
#include <iterator>
// Better return std::vector<char>
char* buildPacket(int code, const std::string& data)
{
auto result = data;
result.append(1, static_cast<char>(code));
char* ret = new char[data.size() + 2];
ret[data.size() + 1] = '\0';
std::copy(result.begin(), result.end(), ret);
return ret;
}
std::vector<char> buildPacketStl(int code, const std::string& data)
{
std::vector<char> ret;
std::copy(data.begin(), data.end(), std::back_inserter(ret));
ret.push_back(static_cast<char>(code));
return ret;
}
int main() {
std::cout << buildPacket(65, "test") << std::endl;; // 65 -> A
auto stl= buildPacketStl(65, "test"); // 65 -> A
std::copy(stl.begin(), stl.end(), std::ostream_iterator<char>(std::cout, ""));
std::cout << std::endl;
}