=（赋值）的例子是右结合的

Question

Where would the associativity of the = assignment operator make a difference in an expression? I thought that the associativity relates to operands that share an operator, but in the case of assignment, how would that work? A few examples that (might) be relevant are:

x = 1
x + 2 = y + 3 = z + 5

Does this just mean that, in the assignments above, we would have:

y + 3 = z + 5

Done before, for example:

x + 2 = y + 3

Or what other scenarios are there where assignment associativity 'matters'?

Answer 1

Your examples don't demonstrate anything, because associativity only comes into play when you have several operators with the same precedence (or the same operator) next to each other.

Consider x = y = 42 , which sets both variables to 42.

Because of right-associativity, it's parsed as x = (y = 42) , where y =... returns the new value of y , which is 42.

This is why it works. If = was left-associative and it was parsed as (x = y) = 42 , then:

• In C it wouldn't compile at all, because x =... returns an rvalue rather than an lvalue, and those can't be assigned to.
• In C++, where assignments return lvalues, it would work like x = y; x = 42; x = y; x = 42; , which is far from being intuitive.