boolean expression in file output

[jijijude]

it is on the use of boolean expression to control the output to file, and this is the code:

bool fileStatus;
...............
...............
ofstream fout(fileName);

fileStatus=fout;

cout<<fileStatus<<endl;
fout<<"information provided by user"<<endl;
fout<<name<<"\n";
fout<<age<<" "<<height<<" "<<weight<<endl;
fout.close();

and the result in the output file does not show the fileStatus thing( cout<<fileStatus<<endl;) and the textbook says this is used as a control to output files. The fout is obviously a class, how can we assign it to the boolean variable? Would anybody please explain how the boolean expression work(control the output to the file)? Thanks!

[slider57]

im a little confused by your question. Are you trying to see if the file opened successfully with the boolean expression so you can begin writing to it?

[andywestken]

The fout is obviously a class, how can we assign it to the boolean variable?

ofstream has an operator void* (inherited from its ios base class)
http://www.cplusplus.com/reference/iostream/ios/operator_voidpt/

And the compiler knows how to treat pointers like booleans, to support code like if (ptr) { ...

so

fileStatus=fout;

is actually

fileStatus=(bool)(void*)fout;

which is the equivalent to

fileStatus=(0 != (void*)fout);

where operator void* is defined along the line of (this is from the Visual C++ xiosbase header)

	operator void *() const
		{	// test if any stream operation has failed
		return (fail() ? 0 : (void *)this);
		}

so you could use

fileStatus=!fout.fail();

instead.

Andy

[jijijude]

Thanks for your clear, neat and thoroughly explanation! I really learn a lot from it. I am sorry I did not make the second question clear. Please see below:

cout<<fileStatus<<endl;

If the fileStatus is 0, will it stop immediately? I mean the

fout<<"information provided by user"<<endl;
fout<<name<<"\n";
fout<<age<<" "<<height<<" "<<weight<<endl;
fout.close();

will not be executed.

In other word, What is the function of the

cout<<fileStatus<<endl;

It seems a little weird to me because it tries to cout a boolean value.

Thanks Andy and Slider!

[andywestken]

As coded, all the calls to insert into fout will still be made, but fail. You need to use an if()

ofstream fout(fileName);

bool fileStatus=fout;

cout<<fileStatus<<endl;
if (fileStatus) {
    fout<<"information provided by user"<<endl;
    fout<<name<<"\n"; 
    fout<<age<<" "<<height<<" "<<weight<<endl;
    fout.close();
}

though you'd normally omit the temp bool variable.

[jijijude]

Yes, thanks for explanation. It makes more sense to me now. Thanks for your kindly help!