在 2012年1月14日星期六UTC+8上午6时48分29秒，Evan Driscoll写道： > On 01/13/2012 03:20 PM, Neil Cerutti wrote: > > They perform the same action, but their semantics are different. > > operator+ will always return a new object, thanks to its > > signature, and operator+= shall never do so. That's the main > > difference I was getting at. > > I was talking about the combination of + and =, since the discussion is > about 'a = a + b' vs 'a += b', not 'a + b' vs 'a += b' (where the > differences are obvious). > > And I stand by my statement. In 'a = a + b', operator+ obviously returns > a new object, but operator= should then go and assign the result to and > return a reference to 'a', just like how 'a += b' will return a > reference to 'a'. >
The operation a+b means add(a,b) and returns a result instance, furthermore a and b can't be modified. The expression a = a+b are two operations not one. But in C or C++ the problem is mixing operations and expressions in a free style allowed. The operation a+=b means a modified by b and b can't be changed. Note that no new instance is necessary in a+=b. > If you're working in C++ and overload your operators so that 'a += b' > and 'a = a + b' have different observable behaviors (besides perhaps > time), then either your implementation is buggy or your design is very > bad-mannered. > > Evan Do you mean the result instances after 'a+=b' and 'a=a+b' or the actions of behaviors of instances involved in performing 'a+=b' and 'a=a+b'? -- http://mail.python.org/mailman/listinfo/python-list
