What does this mean: "class DATA_C data_container"

Question

I'm new to C++ and while I'm reviewing someone else's code, I ran into an expression I don't understand.

In the header file I have a normal class definition:

//in data.hpp
class DATA_C : public QThread
{
   Q_OBJECT
   // the rest of class definition...
}

A snippet of the source file:

//in data.cpp
class DATA_C data_container;

I can understand "DATA_C data", which is a declaration; but what is "class DATA_C data_container"? What does it do?

Thanks in advance.

I'm not 100% on this, but I suspect it's probably something to do with C, and is maybe answered here: https://stackoverflow.com/questions/7729646/using-struct-keyword-in-variable-declaration-in-c — Tas, Sep 06 '18 at 03:30
AFAIK，When instantiating a class object, you can use the Class keyword or not use it. — GeekLee, Sep 06 '18 at 03:35
@eyllanesc this piece of code was written by one of my colleagues — megamonium, Sep 06 '18 at 03:42

score 1 · Accepted Answer · answered Sep 06 '18 at 03:53

In this context tt has exactly the same meaning as DATA_C data_container;. In many contexts you can optionally use the term class X , if it would have been valid to just use X.

In general you can use class X when X has not been defined yet and it declares the class (but doesn't define it). But class DATA_C data_container; would not be allowed if DATA_C had not been previously defined, because you cannot instantiate an object of incomplete type.

What does this mean: "class DATA_C data_container"

1 Answers1