Re: Using the ternary operator to initialize derived class objects

From:

Goran <goran.pusic@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Sun, 30 Oct 2011 22:40:11 -0700 (PDT)

Message-ID:

<c6a9207b-581f-4e65-b317-ccf2fbed6a9b@gk10g2000vbb.googlegroups.com>

On Oct 31, 1:35 am, Dwight Army of Champions
<dwightarmyofchampi...@hotmail.com> wrote:

Hello! Suppose I have two derived classes Car and Bicycle that derive
from a base class Vehicle and an integer x. Why does the following
code give an "operand types are incompatible" error...

Vehicle* AnyVehicle = (x % 2 == 0) ? new Car() : new Bicycle();

...but the following code, which utilizes a simple if statement to do
exactly the same thing, compiles successfully?

Vehicle* AnyVehicle;

if (x % 2 == 0) {
AnyVehicle = new Car();}

else {
AnyVehicle = new Bicycle();

}

Is it even possible to initialize these objects with the ternary
operator?

The problem is, in "left : right" part of the ternary, you have two
unrelated types as far as the compiler is concerned. It tries to
deduct a type for that part, and does not try upcasting (note that
upcasting can get pretty messy if attempted on a more complicated
derivation case). You can use static_cast<Vehicle*> on any side to
work-around. The fact that you have Vehicle on the left does not
matter, as usual conversion rules are being applied on the "="
boundary, so the right side of "=" is done in on it's own.

Goran.