Packing multiple variable into one string

Question

A stream of chars are sent out to a communication device. For readability purposes, i wrote inidividual configurations as variables:

unsigned char a1;
unsigned char a2;
unsigned char a3;
unsigned char a4;
unsigned char a5;
std::string a6;
unsigned char a7;
unsigned char a8;

What is the best way to pack it into a variable tightly so that it's aligned perfectly? Till now I've think of put everything into a struct.

edit: struct doesn't look like a viable option since struct doesn't hold string, and string is varying in size, although is a one time declared thing. Compiled in GCC

edit2: Gonna go with packed struct method, but will convert the string to a c_str first. Until a better answer, this is the way to be.

While there is already a correct answer: Which compiler(s) are you using? Unfortunately, the packing is compiler-specific. — OregonGhost, May 18 '11 at 08:14

score 3 · Accepted Answer · edited May 23 '17 at 10:34

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A packed structure, rather than just a structure would be more appropriate.

EDIT: Ofcourse, you should not use string as a part of your structure, It skipped me while answering but as others have pointed out. You should conver string to character array using str.c_str()); and then store the same in the packed structure.

edited May 23 '17 at 10:34

Community

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answered May 18 '11 at 08:12

Alok Save

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Thanks for the help, but I think some other people has point out that the string has to be in c_str first. – freonix May 18 '11 at 08:44
@freonix. defenitely it has to be c string. for cpp string you do not know the internal structure and it is not appropriate to send it to devices directly. – phoxis May 18 '11 at 08:48

barbacan · Answer 2 · 2011-05-18T08:31:11.790

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You really want to send std::string to device? string has no fixed size. you should use a6.c_str(), and send all with an array of unsigned char and it's size?

edited May 18 '11 at 08:31

answered May 18 '11 at 08:15

barbacan

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Hm... the string should be a char array.Sorry bout that – freonix May 18 '11 at 08:35

score 1 · Answer 3 · answered May 18 '11 at 08:18

struct foo {
   unsigned char a1;
   unsigned char a2;
   unsigned char a3;
   unsigned char a4;
   unsigned char a5;
   .
   .
   .

} __attribute__((packed));

GCC specific. Read: http://gcc.gnu.org/onlinedocs/gcc/Variable-Attributes.html#Variable-Attributes Also have a look at the aligned variable attribute

Or manually insert dummy variables to fillup the places where the compiler may introduce padding.

score 0 · Answer 4 · answered May 18 '11 at 08:14

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A simple array of unsigned char should be enough. But struct is the better way though.

Btw, I don't think you can use std::string in the struct. You need array of unsigned char there.

answered May 18 '11 at 08:14

Donotalo

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score 0 · Answer 5 · answered May 18 '11 at 08:18

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I'm not sure if this is your request but you can use std::ostringstream class to merge all the variables in a single string:

std::ostringstream sOutStream;
sOutStream << a1 << a2 << a3 << a4 << a5 << a6 << a7 << a8;
std::string sOutput = sOutStream.str();

answered May 18 '11 at 08:18

ascanio

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Yea and that too, but I was going to do two things at once, trying to go for speed. Doing a serialization seems to drag a bit of more time, but of course, its safer. – freonix May 18 '11 at 08:45

score 0 · Answer 6 · answered May 18 '11 at 08:35

If you want to send an object like a std::string you'll need to use some sort of serialization and/or protocol. The 'raw' string object will contain typically contain a pointer to the actual string data - and that pointer will be meaningless on the other end of the link. Not mention that the data the pointer points to won't be sent (without some sort of serialization) and that some sort of protocol will be necessary to deal with the variable length data that a std::string represents.

I think you may have more to consider than just packing things tightly.

I was going for the easier way out and trying to cutdown some processing time by doing two things at one. — freonix, May 18 '11 at 08:39

Packing multiple variable into one string

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