在C ++中，传递指针仍会复制对象吗？

Question

I've been reading for an hour now and still don't get what is going on with my application. Since I am using instances of object with new and delete , I need to manage the memory myself. My application needs to have long uptimes and therefore properly managing the memory consumption is very crucial for me.

Here's the static function I use to dump the datapacket, which is transfered between a PC and the I/O board in both directions. The datapacket is an array of BYTE s and is encapsulated into an object, either DCCmd or DCReply (both are implementation of an abstract DCMessage class).

void DebugTools::dumpBytes(BYTE* bytes, short length)
{
    printf("       |---DUMPING DATAPACKET refId: %d ....\n", &bytes);
    for(short i=0; i<length; i++){
        printf("       | B%d | %.2X\n", i, bytes[i]);
    }
    printf("       |---END DUMP           refId: %d ....\n", &bytes);
}

Then there's this use case: I create a DCCmd object and add it to the outgoing message queue to be sent. The "pump" (an infinite loop) checks the outbox and passes any candidates to a IOConnector singleton object.

DCCmd* cmd = new DCCmd(DIG_CMD_SELFTEST_RES);
cmd->add(param);
printf("cmdSelfTest()\n"); //HACK
BYTE* cmda = cmd->getBytes(); //HACK
DebugTools::dumpBytes(cmda, cmd->getLength()); //HACK
sendMsg(cmd);

... and adding to the queue:

bool DC::sendMsg(DCMessage* msg)
{
    if(isOnline()){
        outbox->add(msg);
        return true;
    } else {
        return false;
    }
}

Adding to the queue is done with void add(DCMessage* msg);

(In the connector class there's another of those dumpBytes() to see what is really going to be sent)

But here's the output:

TESTING MESSAGE QUEUES ....
cmdSelfTest()
       |---DUMPING DATAPACKET refId: 2489136 ....
       | B0 | C6
       | B1 | A1
       | B2 | 00
       | B3 | 01
       | B4 | 10
       | B5 | 00
       | B6 | 01
       | B7 | 78
       |---END DUMP           refId: 2489136 ....
    adding to queue: 2488884
   queues: inbox (0), outbox (1)
send: sending candidates....
  sending 2489164 ....
    >->-> ...
       |---DUMPING DATAPACKET refId: 2488704 ....
       | B0 | C6
       | B1 | A1
       | B2 | 00
       | B3 | 01
       | B4 | 10
       | B5 | 00
       | B6 | 01
       | B7 | 78
       |---END DUMP           refId: 2488704 ....
Packet sent!
. ((second iteration of the pump))
   queues: inbox (0), outbox (1)
send: sending candidates....
  sending 2489164 ....
    >->-> ...
       |---DUMPING DATAPACKET refId: 2488704 ....
       | B0 | C6
       | B1 | A1
       | B2 | 00
       | B3 | 01
       | B4 | 10
       | B5 | 00
       | B6 | 01
       | B7 | 78
       |---END DUMP           refId: 2488704 ....
Packet sent!

Can someone please shed some light why the references are different each time I pass from one block to the other? What does this mean to the memory consumption? How can I make sure I am not duplicating memory? Thanks.

Answer 1

The variable bytes is the pointer to the data, ie the memory location of the data. But that is not what you are printing, you are printing out the address where this pointer is located, ie the address on the stack where the pointer is passed. So

printf("       |---DUMPING DATAPACKET refId: %d ....\n", &bytes);

should just be

printf("       |---DUMPING DATAPACKET refId: %d ....\n", bytes);

Answer 2

bytes变量是dumpBytes中的一个函数参数，在本例中为指针，您会从传递的指针中复制一个新指针，但它仍然是一个新指针，在堆栈中有自己的地址 ，因此取其地址将每次都不同，除非它是从同一位置调用的，并且堆栈地址由于纯巧合而导致相同。

Answer 3

In your calls to dumpBytes you passed the bytes using pass-by-copy instead of pass-by-reference.

This results in a new pointer to BYTES being created for the lifetime of the dumpBytes. Depending on your system this will be 8,16,32,64 bytes etc. In otherwords, unless you've got really, really tight memory constraints then it's not an issue.