Trouble understanding pass by reference

问题

I find it really confusing to understand pass by reference in c#. In my code I have function which takes two parameters

private bool SerialUnEscape(byte serialData, ref byte serialResult)
    {
        if (((SerialProcessValue)serialData == SerialProcessValue.SERIAL_PACKET_START) ||
                     ((SerialProcessValue)serialData == SerialProcessValue.SERIAL_PACKET_END)) {
            serialEscaped = false;
            serialResult = 0;
            return (true);
            }

        } else if (serialEscaped) {

            if ((SerialProcessValue)serialData == SerialProcessValue.SERIAL_PACKET_ESCAPE_START) {
                serialResult = (int)SerialProcessValue.SERIAL_PACKET_START;
                serialEscaped = false;
                return (true);
            } 
        } else {
            serialResult = serialData;
            return (true);
        }

    }

I am calling the function with a reference as serialDataLsb & serialDataMsb.

Now my confusion is about, what would be the value of serialDataLsb or serialDataMsb, Does it get the value of serialResult ??

  for (i = 0; i < serialElements; i++) {
    serialDataLsb = 0;
    serialDataMsb = 0;
    while (serialBufferWalk < serialIndex) {
        if (SerialUnEscape(serialBuffer[serialBufferWalk++], ref serialDataLsb)) {
            break;
                }
               }
    while (serialBufferWalk < serialIndex) {
        if (SerialUnEscape(serialBuffer[serialBufferWalk++], ref serialDataMsb)) {
            break;
            }
        }
serialElementData[i] = (UInt16)(serialDataLsb + (serialDataMsb << 8));
}

and i need to port this code into python, 1) how can i implement the pass by reference in python

I tried using this

while serialBufferWalk < serialIndex:
    if self.SerialUnEscape(serialBuffer[serialBufferWalk += 1],serialDataLsb):
        break
while serialBufferWalk < serialIndex:
    if self.SerialUnEscape(serialBuffer[serialBufferWalk += 1],serialDataLsb):
        break

回答1:

If you really want to imitate pass-by-reference instead of changing your code to return the value, you can wrap the primitive you want to change in an object. For simplicity, I used a list:

def change_reference(byteContainer):
    byteContainer[0] = 42

b = 123
print(b) # Prints 123

# Copy the data into the container list.
container = [b]
# Pass a pointer to that list into the function.
change_reference(container)
# Take the value out of the container.
b = container[0]

print(b) # Prints 42

This makes your function really confusing though. What you should really do is include the modified byte in the return value:

def test_and_subtract(v):
    if v == 1:
        return (v - 1, True)
    return (v - 2, False)

v = 1
v, b = test_and_subtract(v)
print(v) # 0
print(b) # True

v = 5
v, b = test_and_subtract(v)
print(v) # 3
print(b) # False

Here return (v - 1, True) is putting both results into a tuple, and b, v is removing them from that tuple.

来源：https://stackoverflow.com/questions/31728934/trouble-understanding-pass-by-reference

标签

python

pass-by-reference