"&/*parent*/" as argument?

pdsc_dy

I found the following sample coding from Qt documentation. Can you please explain the highlighted part? I am trying to understand the syntax for the argument: (const QModelIndex &/parent/). The /parent/ should be just comment, so the argument is just "const QModelIndex &"?

int DomModel::columnCount(const QModelIndex &/*parent*/) const
{
    return 3;
}

mrjj

@pdsc_dy
its a not so good way of suppressing "unused parameter" warning.

so yes its just
"const QModelIndex &"

pdsc_dy

What does the "&" stand for in this case? Thanks!

hskoglund

Hi, it's a reference (a fancier type of pointer, which allows you to write parent.somevar instead of parent->somevar)

mrjj

@pdsc_dy
One thing about references that makes them different from pointers.
They cannot be null

void somefunc1( someting &var )

void somefunc2( someting * var)

somefunc1(NULL) ;// will give error

somefunc2(NULL) ;// accepted

Chris Kawa

@mrjj said:

They cannot be null

Sure they can: int& bar = *(int*)nullptr; - of course it's useless, dereferencing crashes the app and it generally makes no sense but there you have it - compiling just fine :)

mrjj

@Chris-Kawa
Heh. Ok. Fair point. If you really want to. :)

jsulm

@pdsc_dy I moved your question to "C++ Gurus" forum as it is not related to Qt but to C++.
References (this & thing) are one of the C++ concepts, so you should learn C++.