c++ assign value to a const QList in if... else

Question

I would like to assign a value to a const QList depending on the value of a pointer.

invar is a pointer, if it is NULL I want to assign a first value to const QList mylist, if it is not NULL, another value. But const correctness is observed: i do not intend to change value for mylist afterwards.

Is it possible to have

if(invar)
    const QList<Type> zoneList = invar->getZones();
else
    const QList<Type> zoneList = aList ;

I am defining zoneList once exactly, so why would this not be working ?

ALso, as I am using zoneList in the next part of my method, I get this compile error: undeclared identifier.

But I am not allowed to write that either

    const QList<Type> zoneList ;

    if(invar)
        zoneList = invar->getZones();
    else
        zoneList = NULL ;

and I have compiler error Error 1

error C2678: binary '=' : no operator found which takes a left-hand operand of type 'const QList<T>' (or there is no acceptable conversion)  

Is it possible to have two different values assigned to this const QList depending on a condition ? What is the right way to write it then ?

Answer 1

Use the ternary operator ?:

const QList<Type> zoneList(invar ? invar->getZones() : aList);

BTW is it NULL or is it aList? You have both in your code above.

This

if(invar)
    const QList<Type> zoneList = invar->getZones();
else
    const QList<Type> zoneList = aList ;

doesn't work because each zoneList variable is defined inside the if statement only.

Answer 2

With such a simple condition, you could just use the ternary operator:

const QList<Type> zoneList = invar ? invar->getZones() : aList;

If the initialization gets more complex than that and you happen to use C++11, then you can use a new idiom made possible by lambdas:

const QList<Type> zoneList = [] () -> QList<Type> {
                          // ^^ possibly [&] to capture variables

        // some complex stuff that returns a QList<Type>

    }(); // Note the () that calls the lambda immediately

If you are stuck with C++03 (or, Stroustrup forbid, C++98), there are few solutions left except calling another function/method, with the drawback that you have to pass as a parameter any local variable you need to compute the const value.

---

Edit: I realize I haven't fully answered your question. Others have already explained that your first if can't work because it's a matter of scope, I won't add much except that adding braces will probably make it clearer for you (note how this is strictly the same code as yours: the braces are optional when the if/else bodies are just a single statement but their presence/absence doesn't change the semantics of the code):

if (invar) {
    const QList<Type> zoneList = invar->getZones();
}
else {
    const QList<Type> zoneList = aList;
}

Answer 3

If you do

if(invar)
    const QList<Type> zoneList = invar->getZones();
else
    const QList<Type> zoneList = aList ;

the scope of zoneList is limited to the if or else branch, and outside it does not exist anymore.
And if you do

const QList<Type> zoneList ;

if(invar)
    zoneList = invar->getZones();
else
    zoneList = NULL ;

zoneList is declared const and cannot be changed in if/else.

For a hack you could use const_cast, but it is bad practice so you must initialize zoneList.

zoneList = invar != 0 ? invar->getZones() : 0; //don't use NULL in C++