how make cast from ifstream to istream in MS Visual C++

[ragi]

Short version of my program:

ifstream File;

File.open("test.txt");
if(!File.good()) return;

Func(File); <--cannot convert parametr 1 from 'std::ifstream' to
'std::istream'


and declaration is:

bool Func(std::istream);


What should I do to solve these problem ?

 

[John Harrison]

Short version of my program:

ifstream File;

File.open("test.txt");
if(!File.good()) return;

Func(File); <--cannot convert parametr 1 from 'std::ifstream' to
'std::istream'


and declaration is:

bool Func(std::istream);


What should I do to solve these problem ?

bool Func(std::istream&);

Always pass streams by reference (or pointer) they cannot be copied.

john

 

[Jonne Lehtinen]

bool Func(std::istream&);

Always pass streams by reference (or pointer) they cannot be copied.

john

If a function is guaranteed to not to modify the passed variables then
they should be passed as const reference. Personally I rarely pass
anything by value since it's usually faster to pass by (const)
reference - especially with classes. I only use by value if the original
variable shouldn't be changed and the function would change it if it's
passed by reference (or pointer, though personally I try to avoid them
at all cost .

- Jonne Lehtinen

 

[Chris \( Val \)]

| John Harrison wrote:
| > | >
| >>Short version of my program:
| >>
| >>ifstream File;
| >>
| >>File.open("test.txt");
| >>if(!File.good()) return;
| >>
| >>Func(File); <--cannot convert parametr 1 from 'std::ifstream' to
| >>'std::istream'
| >>
| >>
| >>and declaration is:
| >>
| >>bool Func(std::istream);
| >>
| >>
| >>What should I do to solve these problem ?
| >
| >
| > bool Func(std::istream&);
| >
| > Always pass streams by reference (or pointer) they cannot be copied.
| >
| > john
| >
| >
|
| If a function is guaranteed to not to modify the passed variables then
| they should be passed as const reference. Personally I rarely pass
| anything by value since it's usually faster to pass by (const)
| reference - especially with classes. I only use by value if the original
| variable shouldn't be changed and the function would change it if it's
| passed by reference (or pointer, though personally I try to avoid them
| at all cost .

In general, you are right, but in this instance
you are wrong.

You cannot declare the istream as an 'const' parameter.

Cheers.
Chris Val

 

[Mike Wahler]

| John Harrison wrote:
| > | >
| >>Short version of my program:
| >>
| >>ifstream File;
| >>
| >>File.open("test.txt");
| >>if(!File.good()) return;
| >>
| >>Func(File); <--cannot convert parametr 1 from 'std::ifstream' to
| >>'std::istream'
| >>
| >>
| >>and declaration is:
| >>
| >>bool Func(std::istream);
| >>
| >>
| >>What should I do to solve these problem ?
| >
| >
| > bool Func(std::istream&);
| >
| > Always pass streams by reference (or pointer) they cannot be copied.
| >
| > john
| >
| >
|
| If a function is guaranteed to not to modify the passed variables then
| they should be passed as const reference. Personally I rarely pass
| anything by value since it's usually faster to pass by (const)
| reference - especially with classes. I only use by value if the original
| variable shouldn't be changed and the function would change it if it's
| passed by reference (or pointer, though personally I try to avoid them
| at all cost .

In general, you are right, but in this instance
you are wrong.

You cannot declare the istream as an 'const' parameter.

Well, you can, but that makes the stream of limited utility
(i.e. "no fishing in that stream!" ). However one can
still call its const member functions (e.g. 'good()', 'eof()',
etc.)

-Mike

 

[Old Wolf]

If a function is guaranteed to not to modify the passed variables then
they should be passed as const reference. Personally I rarely pass
anything by value since it's usually faster to pass by (const)
reference - especially with classes. I only use by value if the original
variable shouldn't be changed and the function would change it if it's
passed by reference (or pointer, though personally I try to avoid them
at all cost .

It is difficult to use a stream by const reference -- the operations
of reading and writing both require the stream to be modified.
It would be rare to pass a stream to a function where the function
did not modify the stream.

 

[Chris \( Val \)]

|
| | >
| > | > | John Harrison wrote:
| > | >
[snip]

Hello Mike, hope you're well

| > | > bool Func(std::istream&);
| > | >
| > | > Always pass streams by reference (or pointer) they cannot be copied.
| > | >
| > | > john
| > | >
| > | >
| > |
| > | If a function is guaranteed to not to modify the passed variables then
| > | they should be passed as const reference. Personally I rarely pass
| > | anything by value since it's usually faster to pass by (const)
| > | reference - especially with classes. I only use by value if the original
| > | variable shouldn't be changed and the function would change it if it's
| > | passed by reference (or pointer, though personally I try to avoid them
| > | at all cost .
| >
| > In general, you are right, but in this instance
| > you are wrong.
| >
| > You cannot declare the istream as an 'const' parameter.
|
| Well, you can, but that makes the stream of limited utility
| (i.e. "no fishing in that stream!" ). However one can
| still call its const member functions (e.g. 'good()', 'eof()',
| etc.)

You're right, though I should have qualified exactly
what I meant.

I meant that even reading of the stream required
an mutable operation to take effect, therefore
declaring the stream as 'const' was not the right
thing to recommend, in this instance.

Cheers.
Chris Val

 

[Jonne Lehtinen]

It is difficult to use a stream by const reference -- the operations
of reading and writing both require the stream to be modified.
It would be rare to pass a stream to a function where the function
did not modify the stream.

Okay, true
Maybe I was generalizing a bit too much...

- Jonne Lehtinen