C++ question - passing char * to fstream fails -- how to do it right ?

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I like to pass char* parameter (file ) to a function and use the file variable to create object type fstream.

int C_Utility::Sub_file_open (char *file, char *option) {
	fstream file; //create object of fstream class
        file.open(file, ios::in);
...

The above code gives me

file.open(file, ios::in);

std::fstream file’ shadows a parameter
  fstream file;  ...

What I have tried:

fstream wants char , so string does not work either.

Posted 18-Feb-20 10:18am

Vaclav_

Updated 18-Feb-20 10:55am

v2

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phil.o · Accepted Answer · 2020-02-18T10:30:00

Solution 1

Do not give the same name to the parameter of the method and the inner variable; you named both file, use distinct names instead.

Posted 18-Feb-20 10:30am

phil.o

Updated 18-Feb-20 10:31am

v2

Comments

Vaclav_ 18-Feb-20 16:38pm

So how do I specify that I want to work on parameter (file) passed ?

Stefan_Lang 19-Feb-20 2:30am

You can't! You must use a different name for your local variable. I thought the solution was already pretty clear on that.

CPallini 18-Feb-20 16:48pm

5.

CPallini · Accepted Answer · 2020-02-18T10:48:00

Quote:
So how do I specify that I want to work on parameter (file) passed ?

C++

int C_Utility::Sub_file_open (char *filename, char *option) 
{
  fstream file; //create object of fstream class
  file.open(filename, ios::in);
  //..

or, if you prefer

C++

int C_Utility::Sub_file_open (char *filename, char *option) 
{
  ifstream file(filename); //create object of fstream class
  //..

Richard MacCutchan · Accepted Answer · 2020-02-18T10:56:00

Solution 3

C++

file.open(file, ios::in);

You have named your fstream file, and the first parameter, which should be a char* (see basic_fstream Class | Microsoft Docs[^]) you have also named file. So the compiler thinks you are trying to pass a fstream object to the open method of a fstream, which makes no sense.

Don't use names like file for anything as it can easily conflict with actual types. Use sensible meaningful names, it will save you headaches in future.

Posted 18-Feb-20 10:56am

Richard MacCutchan

Comments

Vaclav_ 18-Feb-20 20:17pm

Apparently fstream OPENS the file hence I was actually doing it twice and that
helped" to create the conflict. Using "shadowing" instead of "duplicate " helps with my vocabulary , however I would prefer "duplicate"....

Stefan_Lang 19-Feb-20 2:48am

What do you even mean by 'duplicate' and 'shadowing'? I have a strong feeling that you are trying to use concepts from another language in C++ which doesn't have that concept!? (Also, the last line of code in your OP uses a ' like a comment sign - but in C++ it does not have that meaning!)

In case the three solutions already offered haven't been sufficiently clear:
In C/C++, when you declare a variable with a specific name, all future (i. e. later in the code) references of that name will refer to that specific variable! If you already had a variable using that name, e. g. a function parameter, that object will be unaccessible throughout the rest of your code!

It is a general rule in C/C++, that you should _never_ reuse a name! Even though under certain circumstances the compiler will allow the redefinition of a symbol, doing that never serves a sensible purpose. (and if you think it does, then either you are simply wrong, or you may be thinking of a concept that does not exist in C++)

tl;dr: Do not reuse variable names in C++! Ever.*

* P.S: this applies to a single function - you can of course use the same name in a different function. (Technically, scope, not function, but that's already a bit dangerous)

Richard MacCutchan 19-Feb-20 3:35am

You forgot to mention scope rules.

Stefan_Lang 19-Feb-20 5:06am

I skipped them deliberately: Even though you can reuse names temporarily within a limited scope, doing so is just asking for trouble. If there is an actually resonable use case for that, I haven't heard of it.

Richard MacCutchan 19-Feb-20 5:14am

That's not what I meant. Your comment implies that if you use a name for a variable anywhere in your program you should never use the same name again. But it is OK to use the same name in different scopes. Although there are plenty of people who don't fully understand the concept.

Stefan_Lang 19-Feb-20 7:31am

Ah, of course. I somehow always implied function scope without saying it. Modifed my comment to clarify that.

Of course, as you say, you can reuse names in different scopes within a function, but you have to be careful. My advice was on the 'better be safe than sorry' line, not a literal prohibition.

Richard MacCutchan 19-Feb-20 7:36am

And, of course, your advice is very good advice. I was trying to be facetious, but probably lost in translation.

Richard MacCutchan 19-Feb-20 3:33am

Not strictly true. It will open the file if you pass the filename into the constructor, but if not you will need to call open.

Vaclav_ 19-Feb-20 7:16am

I like to add this "comment".
I have asked "how to do it right ..."
CPallini's is the best solution , and with combination of last Richard's comment actually explains why - irregardless of my mistake in using same names - creating ifstream object or creating ifstream object with parameter is the main difference . Thanks for all the comments - SOLVED and more.