Why Do C Copy Assignment Operators Return a Reference (or Const Reference)?

Why is Returning a Reference/Const Reference Necessary for the Copy Assignment Operator?

In C , the copy assignment operator typically returns a reference or a const reference to itself. This design choice stems from performance considerations and avoids unnecessary copying.

Consider the following example:

A a1(param);
A a2 = a1;
A a3;

a3 = a2;

Suppose the copy assignment operator operator= returned a copy of the new object. In this case, each assignment operation would trigger the creation of a temporary copy and the subsequent destruction of that copy after the assignment.

A a3 = a2; // Creates a temporary copy of a2

However, by returning a reference or a const reference, the assignment operation becomes much more efficient. The data is copied directly from one object to another, without the need to create and destroy temporary copies.

A& operator=(const A& a) { /* ... */ }
a3 = a2; // Copies data directly from a2 without creating a temporary copy

Returning a reference or a const reference also ensures that the operator can be chained. In the example above, the statement a = b = c would work correctly because the assignment operator returns a reference to the calling object.

In contrast, if the operator returned a copy, chaining would not be possible. The statement a = b = c would produce a compilation error because the assignment of c to a would require a copy, while the assignment of b to a already created a copy.

Therefore, returning a reference or a const reference from the copy assignment operator is essential for both performance and the correct chaining of multiple assignments.

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