How do I write an int to a stringstream?

Question

I have std::stringstream with 1 byte:

std::stringstream message;
message.write((const char[]) {0x55}, 1);
std::string res(message.str());

How append int a = 1; (4 bytes)?

I want to get as values stored in the message's underlying std::string:

0x55 0x00 0x00 0x00 0x01

And right tool I chose for the job with the sequence of bytes?

Why do you want to get `0x55 0x00 0x00 0x00 0x01`? Are you sure you want to treat it as a byte buffer instead of a textual string? Just asking to make sure — AliciaBytes, Jun 17 '15 at 19:44
This may be of some help (I'm not flagging as duplicate because I'm honestly not sure): http://stackoverflow.com/questions/191757/c-concatenate-string-and-int — Ricky Mutschlechner, Jun 17 '15 at 19:44
`std::string` is not `std::stringstream`, finding out how to use `std::stringstream` is easy, and your syntax is made-up. Which book are you using? — Lightness Races in Orbit, Jun 17 '15 at 19:44
Why is your title talking about `std::string`? The code you've shown is not valid C++, it's using C's compound literals which your compiler allows as an extension. — Praetorian, Jun 17 '15 at 19:47
@comm1x _"I want to get: `0x55 0x00 0x00 0x00 0x01`"_ As content or output? — πάντα ῥεῖ, Jun 17 '15 at 19:48
@RaphaelMiedl I need to implement specific socket protocol where 2-5 bytes specify length of next message body — Pavel Shorokhov, Jun 17 '15 at 19:48
@πάνταῥεῖ as a content. After I send It to server. — Pavel Shorokhov, Jun 17 '15 at 19:48

score 3 · Accepted Answer · edited May 23 '17 at 12:05

3

"How append int a = 1; (4 bytes)?"

Simply like that:

uint32_t a = 1; // Use uint32_t to be sure to have 4 bytes.
message.write((const char*)&a, sizeof(a));

As you mention "specific socket protocol", you'll probably need to take care of the network byte order using the htonl() function:

uint32_t a = htonl(1);

edited May 23 '17 at 12:05

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answered Jun 17 '15 at 19:51

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stj · Answer 2 · 2015-06-17T20:08:19.080

The following will do, with function Append4ByteInteger doing the actual work:

#include <iostream>
#include <sstream>
#include <string>

static std::stringstream& Append4ByteInteger (std::stringstream& stream, int value) {
  auto v = static_cast<uint32_t>(value);

  stream << static_cast<unsigned char>(v >> 24);
  stream << static_cast<unsigned char>(v >> 16);
  stream << static_cast<unsigned char>(v >> 8);
  stream << static_cast<unsigned char>(v);

  return stream;
}

int main () {
  std::stringstream message;
  message.write((const char[]) {0x55}, 1); 

  Append4ByteInteger(message, 1);
  std::cout << "result: '" << message.str() << "'" << std::endl;
}

You can check that the stream actually contains 0x00 0x00 0x00 0x01 using a hex dump utility (e.g. hexdump).

Note that the function assumes that the integer has a value of 0 or greater. If you want to ensure that, you may want to use an unsigned int instead, or probably even a uint32_t, as the size of an int is platform-dependent.

You forgot to right shift before inserting into the stream. – Praetorian Jun 17 '15 at 19:54 — Praetorian, Jun 17 '15 at 19:54
Absolutely. Thanks, I'll fix my answer! – stj Jun 17 '15 at 20:03 — stj, Jun 17 '15 at 20:03

score 0 · Answer 3 · answered Jun 17 '15 at 19:54

Writing an int might differ based on internal byte order, big endian, little endian, or others.

It would be more portable to mask off each byte logically, and then put each unsigned char into the stream

You will need to rebuild the int on the recieving side, again using logical operations like shift

It is also likely you will not need all 4 bytes, so you could easily just send 3 for example.

You should look up shifting and masking and understand them if you plan to do binary streams.

How do I write an int to a stringstream?

3 Answers3