Variadic template: error: parameter packs not expanded with '...'

Question

I'm trying to pass multiple strings to fill a Container, but I receive this error. Using gcc 4.9.3

template< class T >
struct DataCompare {
    bool operator()( const T& lhs, const T& rhs ) const
    {
        return operator<( lhs->getCode(), rhs->getCode() );
    }
};

using AggContainer = boost::container::flat_set< T, C >; 
using DataPtr      = boost::shared_ptr< BomQueueData >;
using PropertyQueueDataLess = DataCompare< DataPtr >;
using QueueDataPtrs = AggContainer< DataPtr, DataLess >;

QueueDataPtrs vector_name;

template< class Container, typename ... Args >
static void fillWithData(Container & oDataContainer, Args const & ... args)
{
    typedef typename Container::value_type::element_type QueueDataPtr;
    oDataContainer.emplace(new QueueDataPtr(args));
}

fillWithData(vector_name, x, a, b, c, d); // compiler error

How to solve?

Please try to create a [Minimal, **Complete**, and Verifiable Example](http://stackoverflow.com/help/mcve) and show us. And also please copy-paste the full error output (long as it might be with templates) as text into the question. — Some programmer dude, Mar 03 '17 at 08:22

score 6 · Accepted Answer · answered Mar 03 '17 at 08:25

6

args is a parameter pack, not a parameter. That's why you can't use:

DataContainer.emplace(new QueueDataPtr(args));

Instead, use

DataContainer.emplace(new QueueDataPtr(args...));

That expands the parameter pack.

answered Mar 03 '17 at 08:25

R Sahu

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1

@rh0x, only if I had a penny for every stupid mistake I committed :) – R Sahu Mar 03 '17 at 08:31
@Sahu ^^ how can I guarantee I'm using the move constructor? I have to define it into BomQueueData? – rh0x Mar 03 '17 at 08:37
1

I think the syntax for that is `QueueDataPtr(std::forward(args)...)` – R Sahu Mar 03 '17 at 08:39

score 3 · Answer 2 · answered Mar 03 '17 at 08:51

For perfect forwarding, use a universal reference for the args parameter, and then forward it:

template< class Container, typename ... Args >
static void fillWithData(Container & oDataContainer, 
                         Args&& ... args)  // universal reference
{
    typedef typename Container::value_type::element_type QueueDataPtr;
    oDataContainer.emplace(new QueueDataPtr(std::forward<Args>(args)...));
}

Variadic template: error: parameter packs not expanded with '...'

2 Answers2