Tcp protocol prefix the length of message - c++

Question

I'm trying to prefix the length of message so its 4 bytes and use big-endian in C++, but i don't understand how to do that. In Node.JS i'm doing it like that:
var buffer = new Buffer("HELLO WORLD");
//create a buffer with +4 bytes
var consolidatedBuffer = new Buffer(4 + buffer.length);
//write at the beginning of the buffer, the total size
consolidatedBuffer.writeInt32BE(buffer.length, 0);
//Copy the message buffer to the consolidated buffer at position 4 (after the 4 bytes about the size)
buffer.copy(consolidatedBuffer, 4);
i'm trying to achieve the same result in C++, but don't know how to do the same, can someone please show me how that is done in C++? the result should be string byte array
I'm doing that on Mac:
std::string data = "HELLO WORLD";
uint32_t length = htonl( data.length() );
std::string payload;
payload.append(std::to_string(length));
payload.append(data);

Answer 1

One way of doing this would be something like
std::vector<char> pack(const std::string& str) {
const uint32_t sz = str.size();
const uint32_t n_sz = htonl(sz);
std::vector<char> result(sizeof(sz) + sz);
memcpy(result.data(), &n_sz, sizeof(n_sz));
memcpy(result.data() + sizeof(sz), str.data(), sz);
return result;
}