problem with pointer-reference

[mickey]

class File {
	private:
		std::fstream file;
       ...............................
     public:
         std::fstream open_file(int);
}
std::fstream File::open_file() {
    file.open(file_name.c_str(), std::ios_base::in); //read only
    return file;
}
 
int readFile(File& f) {
    std::fstream ff = f.open_file(); //EDIT
   if (ff) { //EDIT : if I put if (!f.open_file()) is always 0
       // I need check if file exists
}

class File {
	private:
		std::fstream file;
       ...............................
     public:
         std::fstream open_file(int);
}
std::fstream File::open_file() {
    file.open(file_name.c_str(), std::ios_base::in); //read only
    return file;
}

int readFile(File& f) {
    std::fstream ff = f.open_file(); //EDIT
   if (ff) { //EDIT : if I put if (!f.open_file()) is always 0
       // I need check if file exists
}

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Hi this doens't LINK. anyone could explain me the error? thanks

[e8johan]

If you show us the errors we don't have to try to find them in your code. What does your compiler tell you?

[jacek]

Try adding a semicolon after the class definition.

[mickey]

(semicolon is a cut&past error)
Above I edited the code because there wasn't a a thing.. it isn't LINKING but it compile

microsoft visual studio 8\vc\include\fstream(933) : error C2248: 'std::basic_ios<_Elem,_Traits>::basic_ios' : cannot access private member declared in class 'std::basic_ios<_Elem,_Traits>'
with
[
_Elem=char,
_Traits=std::char_traits<char>
]
microsoft visual studio 8\vc\include\ios(151) : see declaration of 'std::basic_ios<_Elem,_Traits>::basic_ios'
with
[
_Elem=char,
_Traits=std::char_traits<char>
]
This diagnostic occurred in the compiler generated function 'std::basic_fstream<_Elem,_Traits>::basic_fstream( const std::basic_fstream<_Elem,_Traits> &)'
with
[
_Elem=char,
_Traits=std::char_traits<char>
]

[wysota]

I suggest that File::open_file returns a reference to the stream instead of a copy.

[mickey]

I suggest that File:pen_file returns a reference to the stream instead of a copy.

why? it doens't linking............
It works if I declare std::fstream& ff (ff as reference). Could me explain why?
thanks

[wysota]

Because you can't copy streams and you try to do that two times (first you return a copy of the stream from the open function and then you try to assign it to another object, which copies it again. You have to work on references.

[mickey]

class File {
    private:
        std::fstream file;
       ...............................
     public:
         std::fstream& open_file();
}
std::fstream& File::open_file() {
    file.open(file_name.c_str(), std::ios_base::in); //read only
    return file;
}
 
int readFile(File& f) {
    std::fstream* ff = f.open_file(); // here I used '*', heere ERROR
   if (ff) {
      ........................
}

class File {
    private:
        std::fstream file;
       ...............................
     public:
         std::fstream& open_file();
}
std::fstream& File::open_file() {
    file.open(file_name.c_str(), std::ios_base::in); //read only
    return file;
}
 
int readFile(File& f) {
    std::fstream* ff = f.open_file(); // here I used '*', heere ERROR
   if (ff) {
      ........................
}

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Hi I changed it but doens't compile. Now ff isn't a copy! Compiler says that I cant convert
fstream into fstream*. Why? Isn't f.open_file returing a 'reference'???

[wysota]

int readFile(File& f) {
  std::fstream* ff = f.open_file(); // here I used '*', heere ERROR
  if (ff) {
   ........................
}

int readFile(File& f) {
  std::fstream* ff = f.open_file(); // here I used '*', heere ERROR
  if (ff) {
   ........................
}

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ff is now a pointer to std::fstream and not a reference...

Try this:

std::fstream &ff = f.open_file();

std::fstream &ff = f.open_file();

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[mickey]

hi, I just tried it and goes well but I'm finding to understand this:
my function open_file return a reference so I do fstream* pfile = open_file() because I always do this eg: int j; int* pint = &j (your suggest for me is something like

int j; int& pint = &j

int j; int& pint = &j

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And this above doens't compile..
So the returned reference of open_file for me is like &j. Isn't that? or what?
thanks...

[wysota]

"int &" is a "reference to int", whereas int* is a "pointer to int". These are types. Now we also have a "dereference operator" (*) which is a unary operator that returns an object pointed by its argument and an "address operator" (&) which is a unary operator that returns an address of its argument. So "&j" is something completely different than "int&". Same goes for asterisks.

I think it's high time for you to download and read Thinking in C++.