Send 4bytes floating point value through tcp From Dart to C++ - c++

Question

I have a trouble with sending 4 bytes double value through tcp socket from Dart client to c++ server.
Below is flutter(Dart) code.
class DataPacket extends object{
String message = "some";
int ID = 1;
double x = 1.38;
#override
String toString() {
String value = message;
value += getCharCodeStringFromInt(ID);
value += getCharCodeStringFromDouble(x);
return value;
}
Uint8List _getInt16LittleEndianBytes(int value) =>
Uint8List(2)..buffer.asByteData().setInt16(0, value, Endian.little);
String getCharCodeStringFromInt(int value){
Uint8List message_id_list = _getInt16LittleEndianBytes(value);
return getCharCodeStringFromUint8List(message_id_list);
}
String getCharCodeStringFromDouble(double value) {
// List<double> temp = List<double>() ;
// temp.add(value);
// Float32List floatlist = Float32List.fromList(temp);
// Uint8List list = Uint8List.view(floatlist.buffer);
// Uint8List list = binaryCodec.encode(value);
Uint8List list = Uint8List(4)..buffer.asByteData().setFloat32(0, value);
print("Uint8List from double : ${list}");
print("Uint8List length from double : ${list.length}");
print("CharCodeString from double Length :
${getCharCodeStringFromUint8List(list).length}");
return getCharCodeStringFromUint8List(list);
}
String getCharCodeStringFromUint8List(Uint8List list){
String charCodeString = "";
list.forEach((charCode) => charCodeString += String.fromCharCode(charCode));
return charCodeString;
}
}
//Some Class
void sendMessage(){
print(DataPacket().toString().length);
List<int> data = _socket.encoding.encode(DataPacket().toString());
//utf8.encode(DataPacket().toString());
print(data.length);
_socket.add(data);
}
I can parse String and int in c++ by memcpy.
but can't double value.
When I checked the contents of Byte Data and Length,
Uint8List that was gotten from double has lengthened in encoding method of socket.
I mean, length of Uint8List from double was 4 before encoded.
However length of return value(List) becomes 7 after encoding.
so print result of DataPacket().toString().length and data.length is different each other.
I can't parse 7 bytes of float in c++..
Commented lines are ways I tried.
Is there any way?
Thank you.

Answer 1

The problem isn't how you are encoding the float as bytes, but rather what you do to it next trying to convert it to a string.
If you want to send 4 bytes through the socket (assuming you are referring to a dart:io Socket), just use the add method.
Socket s;
var floatValue = 1.38;
var bytes = Uint8List(4)
..buffer.asByteData().setFloat32(0, floatValue, Endian.little);
print(bytes); // prints [215, 163, 176, 63]
s.add(bytes);
In this example, you've started with the target byte array and then used it asByteData to set one value. This is a good way to build up a struct of mixed types - ints, floats, etc. If you just need to convert an array of floats to bytes you can do it slightly more simply with:
var bytes = Float32List.fromList([floatValue]).buffer.asUint8List();
I'm not sure you really need to be doing anything with strings. Is what you really want:
Socket s;
var message = 'some';
var id = 1;
var x = 1.38;
s.add(ascii.encode(message)); // choose the appropriate codec: ascii, utf8
s.add(Int16List.fromList([id]).buffer.asUint8List());
s.add(Float32List.fromList([x]).buffer.asUint8List());
// rather than adding each in turn, you could also form a longer byte array
// of the 3 elements and add that