Why does operator = return *this?

Question

Say I want to override the operator = so I can do something like

Poly p1;  // an object representing a polynomial
Poly p2;  // another object of the         


        

Answer 1

One might be tempted to make the copy-assignment operator return void if you won't ever need chained assignments (as shown in the other answers) anyway. After all, chained assignments are often hard to read and understand, so not allowing them might be considered an improvement.

However, an often overlooked aspect is that void operator=(Poly& const) means that your type would no longer fulfuill the CopyAssignable concept, which requires a T& return type.

A type which does not fulfill the CopyAssignable concept cannot be officially used for some standard-container operations, for example std::vector::insert, which means that the following seemingly innocent piece of code yields undefined behaviour, even though it probably runs perfectly fine:

#include 

struct Poly
{
    void operator=(Poly const&) {} // Poly is not CopyAssignable
};

int main()
{
  std::vector v;
  Poly p;
  v.insert(v.begin(), p); // undefined behaviour
}

As the C++ standard explains in § 17.6.4.8/2.3 where it talks about constraints on programs using the standard library:

(...) the effects are undefined in the following cases:

(...) for types used as template arguments when instantiating a template component, if the operations on the type do not implement the semantics of the applicable Requirements subclause (...).

Of course, it's precisely because of the undefined behaviour that a compiler is allowed to ignore the error and make the program behave nicely, matching the obviously intended behaviour. But it's not required to do so.

You should also consider that you cannot predict all future uses of your Poly type. Someone might write some template function such as:

template 
void f(T const& t)
{
    T t2;
    T t3 = t2 = t;
    // ...
}

This function would then not work with your Poly class.

Just don't violate this C++ convention and you won't run into troubles.