Default ofstream class argument in function

Question

I've got function. If user didn't provide ofstream element into this function:

bool isPolpierwsza(int, bool = false, ofstream = NULL);

than I want to asign "plik"

bool isPolpierwsza(int liczba, bool wypisz, ofstream plik)

to NULL value.

My compiler put error:

2.9.cpp:5:48: error: no viable conversion from 'long' to 'ofstream' (aka 'basic_ofstream')
bool isPolpierwsza(int, bool = false, ofstream = NULL);

How to setup default value to ofstream to be treated like NULL or "false"?

Overload it with a different function. It is C++, after all. so provide two versions; one without the stream (which should be a reference in the first place). — WhozCraig, Oct 19 '13 at 18:27
`ofstream` is not a pointer. You can't set it to `NULL`. The `NULL` macro is most probably defined as `0L`, hence this error. — , Oct 19 '13 at 18:27
There is no such thing as "a null value" generally. Some types (e.g. pointers or optional types) have such a notion, but most types don't. — Kerrek SB, Oct 19 '13 at 18:30

score 2 · Accepted Answer · answered Oct 19 '13 at 19:12

2

You can use two overloads, one of which doesn't take the std::ofstream argument:

bool isPolpierwsza(int liczba, bool wypisz)
{
    return isPolpierwsza(liczba, wypisz, /* your own argument */);
}

answered Oct 19 '13 at 19:12

David G

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What would you use for `your own argument`, in the cases where the intention is not writing anything whenever `wypisz=false`? Something "analogous" to `NULL` seems like the right thing... – sancho.s ReinstateMonicaCellio Sep 06 '20 at 09:57
Yes, you would simply pass `NULL` as the third argument – David G Sep 06 '20 at 15:01
But I think `NULL` cannot be used for an `ofstream`. – sancho.s ReinstateMonicaCellio Sep 06 '20 at 16:22
@sancho.sReinstateMonicaCellio Change the `ofstream` argument to a pointer. (`ofstream*`). – David G Sep 06 '20 at 18:26
@sancho.sReinstateMonicaCellio And you check if the `ofstream` is valid with an if statement: `if (wypisz) { ... }`. And you dereference it like (`*stream`) in order to use it. – David G Sep 06 '20 at 19:13
I guess this part is essential fo the propsal to work. I would add it to the answer. – sancho.s ReinstateMonicaCellio Sep 07 '20 at 00:02
Related: https://stackoverflow.com/questions/63762945/default-null-value-for-an-ofstream – sancho.s ReinstateMonicaCellio Sep 07 '20 at 00:02

score 1 · Answer 2 · answered Oct 19 '13 at 19:15

1

You could pass the stream buffer instead:

bool isPolpierwsza(int, bool = false, std::streambuf* osbuf = nullptr)
{
    std::ostream os(osbuf? osbuf : std::cout.rdbuf());
    os << "yay it works\n";
    return false;
}

Now, call it like this:

std::ofstream ofs("myoutput.txt");
bool result = isPolpierwsza(42, true, ofs.rdbuf());

Or indeed, without the parameter.

answered Oct 19 '13 at 19:15

sehe

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@0x499602D2 Well, this feels more like in the spirit of the IOStream library design (which I know, hurts many people's eyes) – sehe Oct 19 '13 at 19:45

user10F64D4 · Answer 3 · 2013-10-19T19:23:57.700

-2

First of all, it may be useful to use ostream instead of ofstream, as this is more general and considered better practice.

Then, you can simply assign 'cout' instead of 'NULL'. NULL doesn't work because ostream is not a pointer but a struct. This means the struct has to be filled out completely and can not be assigned NULL. Instead, you could use cout, like

bool isPolpierwsza(int, bool = false, ostream& = cout);

edited Oct 19 '13 at 19:23

answered Oct 19 '13 at 18:27

user10F64D4

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This won't even compile. streams cannot be publicly copied by-value. That last parameter needs to be a reference. – WhozCraig Oct 19 '13 at 18:28
1

This will print stuff to standard output; I guess OP wanted not to print stuff at all! – anatolyg Oct 19 '13 at 19:28

Default ofstream class argument in function

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