How to pass variable number of arguments to lambda function

Question

I am trying to pass variable number of arguments to a lambda function. what is the prototype to accept variable number of arguments in lambda functions? should i write a named function instead of lambda?

std::once_flag flag;

template<typename ...Args>
void gFunc(Args... args)
{
}

template<typename ...Args>
void func(Args... args)
{
    std::call_once(flag,[](/*accept variable number of arguments*/... args)
                        {
                                // more code here
                                gFunc( args...);
                        }, 
                        args...
                        );
}

the below signatures give error:

[&](){ }
[&args](){ }
[&args...](){ }
[&,args...](){ }
[&...args](){ }

Anton Savin · Accepted Answer · 2014-10-24 18:41:40Z

12

In C++14 you can do

auto lambda = [](auto... args) {...};

But in your case I believe simple capture is enough:

std::call_once(flag, [&] {
                         gFunc(args...); // will implicitly capture all args
                     }
               );

answered Oct 24, 2014 at 18:41

Anton Savin

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2 Comments

Anton Savin Over a year ago

@ProgramCpp works witgcc 4.9 and clang 3.5

serup Over a year ago

how would you access the arguments inside your auto lambda = [][auto... args) { ... };

P0W · Accepted Answer · 2014-10-24 17:27:51Z

6

Try this :

template<typename ...Args>
void func(Args... args)
{
    std::call_once(flag,[&, args...]( )
                        {   
                         gFunc( args...);
                        }, 
                        args...
                        );
}

Stolen from here

§ 5.1.2 A capture followed by an ellipsis is a pack expansion (14.5.3).

[ Example:
template<class... Args>
void f(Args... args) {
auto lm = [&, args...] { return g(args...); };
lm();
}
—end example ]

answered Oct 24, 2014 at 17:27

P0W

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3 Comments

ProgramCpp Over a year ago

If the argument pack is expanded, how can we access the argument pack again? This signature does not work.

P0W Over a year ago

@ProgramCpp It works with clang and gcc 4.9

ProgramCpp Over a year ago

Thank you :) I would like to pass only args by value [args...](){gFunc(args...);};

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