Low level details of C/C++ assignment operator implementation. What does it return?

Question

I m a total newbie to a C++ world (and C too). And don't know all its details. But one thing really bothers me. It is constructions like : while (a=b) {...} .As I understand this magic works because assignment operator in C and C++ returns something. So the questions: what does it return? Is this a documented thing?Does it work the same in C and C++. Low level details about assignment operator and its implementation in both C and C++ (if there is a difference) will be very appreciated!

I hope that this question won't be closed, because I can't find a comprehensive explanation and good material on this theme from the low level point of view all the more so.

http://stackoverflow.com/questions/4421706/operator-overloading — chris, Feb 04 '13 at 23:28
It returns what was assigned to `a`. Unless the operator was overloaded to return something else. — imreal, Feb 04 '13 at 23:29
@Nick But what about this while (a=b) {...}. While accept bool. And overloading is a C++ feature and what about C? — MainstreamDeveloper00, Feb 04 '13 at 23:31
I would be quite upset if the assignment operator were to return. Which book are you reading? — autistic, Feb 04 '13 at 23:34
@modifiablelvalue I don't know if I misunderstood you, but the signature is mostly always `T &operator =(T &left, const T &right)`, returning `left`. Otherwise you could not use `a = b = c`. — Kijewski, Feb 04 '13 at 23:36
@Kay `returning` in C++ isn't the same as `evaluating` in C. — Aniket Inge, Feb 04 '13 at 23:39
@modifiablelvalue iff `b = c; a = b;` is valid, then the chain expression is valid too. — Kijewski, Feb 05 '13 at 03:27
@Kay I think you've missed my point, which was slightly supplemented by Aniket. — autistic, Feb 05 '13 at 04:04

bames53 · Accepted Answer · 2013-02-05T20:04:10.453

19

For built-in types in C++ evaluating an assignment expression produces an lvalue that is the left hand side of the assignment expression. The assignment is sequenced before the result can be used, so when the result is converted to an rvalue you get the newly assigned value:

int a, b=5;
int &c = (a=b);
assert(&c==&a);
b=10;
assert(10==(a=b));

C is almost but not exactly the same. The result of an assignment expression in C is an rvalue the same as the value newly assigned to the left hand side of the assignment.

int *c = &(a=b); // not legal in C because you can only take the address of lvalues.

Usually if the result of an assignment is used at all it's used as an rvalue (e.g., a=b=c), so this difference between C++ and C largely goes unnoticed.

edited Feb 05 '13 at 20:04

answered Feb 04 '13 at 23:39

bames53

86,085
15
179
244

Appreciate if someone could point me to a reference where this is stated. – KcFnMi Oct 26 '22 at 02:58
@KcFnMi In the C++20 standard it's defined in `7.6.19 [expr.ass] /1`: "their result is an lvalue referring to the left operand. [...] the assignment is sequenced after the value computation of the right and left operands, and before the value computation of the assignment expression." – bames53 Oct 26 '22 at 16:49

score 7 · Answer 2 · answered Feb 04 '13 at 23:50

The assignment operator is defined (in C) as returning the value of the variable that was assigned to - i.e. the value of the expression (a=b) is the value of a after the expression has been evaluated.

It can be defined to be something different (of the same type) for user-defined operator overloads in C++, but I suspect most would consider this to be a very unpleasant use of operator overloading.

You can use this (non-boolean) value in a while (or an if, etc.) because of type conversion - using a value in a conditional context causes it to be implicitly converted to something which makes sense in a conditional context. In C++, this is bool, and you can define your own conversion (for your own type) by overloading operator bool(). In C, anything other than 0 is true.

Aniket Inge · Answer 3 · 2013-02-04T23:41:50.957

To understand such expressions, you have to first understand that, positive integers are considered as 'true' and 0 is considered as false.

An assignment evaluates to the left hand side of the operator = as its value. So, while(a=b) { } would mean, while(1 /*true*/) if a after being assigned to b evaluates to non-zero. Else, it is considered as while(0 /*false*/)

Similarly, with the operator (a=b)?1:0 is the value of a after being assigned to b .. if it is non-zero then the value is taken as true and the statement following ? will be executed, or the statement following : is executed.

Assignments usually evaluate to the value at the left hand side of the operator = where as, logical operators(such as ==, && etc) evaluate to 1 or 0.

Note: with C++, it will depend upon if or not, a certain operator is overloaded.. and it will also depend upon the return type of the overloaded operator.

The ternary operator has a higher precedence than the assignment. `a=b ? 1 : 0` is parsed as `a = (b ? 1 : 0)`. — Kijewski, Feb 04 '13 at 23:39
@Aniket: In C++, the default copy assignment returns a reference to the left side, even if it's not overloaded. — Mooing Duck, Feb 04 '13 at 23:45

score 0 · Answer 4 · answered Feb 04 '13 at 23:41

The assignment operators in C and C++ return the value of the variable being assigned to, i.e., their left operand. In your example of a = b, the value of this entire expression is the value that is assigned to a (which is the value of b converted into the type of a).

So you can say that the assignment operator "returns" the value of its left operand.

In C++ it's a little more complicated because you can overload the = operator with an actual user-defined function, and have it return something other than the value (and type) of the left operand.

Low level details of C/C++ assignment operator implementation. What does it return?

4 Answers4

Linked