使用不同的字段解析消息

我有一个字节流，表示我的应用程序中的消息。消息中有 5 个字段用于演示。流中的第一个字节指示当前流存在哪些消息字段。例如，字节 0 中的0x2表示当前流仅存在字段 1。

掩码字段可能有 2^5=32 个不同的值。为了解析这种不同宽度的消息，我在下面编写了示例结构和解析器。我的问题是，有没有其他方法可以解析这种动态变化的字段？如果消息有 64 个字段，我将不得不写 64 个案例，这很麻烦。

#include <iostream>
typedef struct
{
uint8_t iDummy0;
int iDummy1;
}__attribute__((packed, aligned(1)))Field4;
typedef struct
{
int iField0;
uint8_t ui8Field1;
short i16Field2;
long long i64Field3;
Field4 stField4;
}__attribute__((packed, aligned(1)))MessageStream;
char* constructIncomingMessage()
{
char* cpStream = new char(1+sizeof(MessageStream)); // Demonstrative message byte array
// 1 byte for Mask, 20 bytes for messageStream
cpStream[0] = 0x1F; // the 0-th byte is a mask marking
// which fields are present for the messageStream
// all 5 fields are present for the example
return cpStream;
}
void deleteMessage( char* cpMessage)
{
delete cpMessage;
}
int main() {
MessageStream messageStream; // Local storage for messageStream
uint8_t ui8FieldMask; // Mask to indicate which fields of messageStream
// are present for the current incoming message
const uint8_t ui8BitIsolator = 0x01;
uint8_t ui8FieldPresent; // ANDed result of Mask and Isolator
std::size_t szParsedByteCount = 0; // Total number of parsed bytes
const std::size_t szMaxMessageFieldCount = 5; // There can be maximum 5 fields in
// the messageStream
char* cpMessageStream = constructIncomingMessage();
ui8FieldMask = (uint8_t)cpMessageStream[0];
szParsedByteCount += 1;
for(std::size_t i = 0; i<szMaxMessageFieldCount; ++i)
{
ui8FieldPresent = ui8FieldMask & ui8BitIsolator;
if(ui8FieldPresent)
{
switch(i)
{
case 0:
{
memcpy(&messageStream.iField0, cpMessageStream+szParsedByteCount, sizeof(messageStream.iField0));
szParsedByteCount += sizeof(messageStream.iField0);
break;
}
case 1:
{
memcpy(&messageStream.ui8Field1, cpMessageStream+szParsedByteCount, sizeof(messageStream.ui8Field1));
szParsedByteCount += sizeof(messageStream.ui8Field1);
break;
}
case 2:
{
memcpy(&messageStream.i16Field2, cpMessageStream+szParsedByteCount, sizeof(messageStream.i16Field2));
szParsedByteCount += sizeof(messageStream.i16Field2);
break;
}
case 3:
{
memcpy(&messageStream.i64Field3, cpMessageStream+szParsedByteCount, sizeof(messageStream.i64Field3));
szParsedByteCount += sizeof(messageStream.i64Field3);
break;
}
case 4:
{
memcpy(&messageStream.stField4, cpMessageStream+szParsedByteCount, sizeof(messageStream.stField4));
szParsedByteCount += sizeof(messageStream.stField4);
break;
}
default:
{
std::cerr << "Undefined Message field number: " << i << 'n';
break;
}
}
}
ui8FieldMask >>= 1; // shift the mask
}
delete deleteMessage(cpMessageStream);
return 0;
}