Assignment Is Right Associative

Unlike the other binary operators, assignment is right associative:

int ival, jval;
ival = jval = 0; // ok: each assigned 0

Because assignment is right associative, the right-most assignment, jval = 0, is the right-hand operand of the left-most assignment operator. Because assignment returns its left-hand operand, the result of the right-most assignment (i.e., jval) is assigned to ival.

Each object in a multiple assignment must have the same type as its right-hand neighbor or a type to which that neighbor can be converted (§ 4.11, p. 159):

int ival, *pval; // ival is an int; pval is a pointer to int
ival = pval = 0; // error: cannot assign the value of a pointer to an int
string s1, s2;
s1 = s2 = "OK";  // string literal "OK" converted to string

The first assignment is illegal because ival and pval have different types and there is no conversion from the type of pval (int*) to the type of ival (int). It is illegal even though zero is a value that can be assigned to either object.

On the other hand, the second assignment is fine. The string literal is converted to string, and that string is assigned to s2. The result of that assignment is s2, which has the same type as s1.