How to apply operator << (or >>) to a parameter pack in C++?

Question

Imagine I have this function template:

template<auto& ... FLAGS>
void function(const char *c_str) {
    std::stringstream strs;
    //((strs << FLAGS)...); // <-- how do I do this?
    strs << c_str;
    .
    .
    .
}

Consider that the function template is going to be used like this:

function<std::dec, std::left>("123");

how to apply the operator << on each parameter from the parameter pack (with the object strs )?

I have already found a workaround with introducing a helper function:

void unpack(const auto& ...){};

And in the original function template:

unpack((strs << FLAGS)...);

But I was wondering if there is a better way to this!

[`(strs << ... << FLAGS)`](http://coliru.stacked-crooked.com/a/d99e8cb9dcefd819). Form (4) on the cppreference page on [fold expressions](https://en.cppreference.com/w/cpp/language/fold). — Miles Budnek, May 29 '23 at 05:40
also peek here https://stackoverflow.com/questions/55763096/fold-expressions-and-cout — Swift - Friday Pie, May 29 '23 at 05:51
@MilesBudnek thanks. That solved the problem and very nicely so! — newbie, May 29 '23 at 05:54

score 1 · Accepted Answer · answered May 29 '23 at 06:00

The expression

strs << FLAG1 << FLAG2 << FLAG3;

is equal to

(((strs << FLAG1) << FLAG2) << FLAG3);

You have to chain operators, therefore you need the form of fold-expression

( cast-expression fold-operator ... fold-operator cast-expression )

where fold-operator is stringstream::operator<<

(strs << ... << FLAGS);

score 1 · Answer 2 · answered May 29 '23 at 06:48

1

Instead a fold with the << operator, you could also fold with the , operator, which would produce something like

str << FLAG1;
str << FLAG2;
...

(actually you'd need to use , instead of ; but the resulting logic is the same).

template<auto& ... FLAGS>
void function(int c_str) {
    std::stringstream strs;
    ((strs << FLAGS), ...);
    //              ^
    strs << c_str;
}

answered May 29 '23 at 06:48

fabian

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Doesn't work with some flags which require chaining – Swift - Friday Pie May 29 '23 at 07:51

How to apply operator << (or >>) to a parameter pack in C++?

2 Answers2