C++ why the assignment operator should return a const ref in order to avoid (a=b)=c

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(x=y) means x.operator=(y), which returns the object x. Therefore, (x=y)=z means (x.operator=(y)).operator=(z). The expression in parens sets x to y and returns x, and then the outer bit sets x to z. It does not set y to z as you might expect, and as the expression x = y = z does.

This behavior is counter-intuitive (they should all be equal after the assignment, right?); returning a const reference makes it impossible and avoids the problem.

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