Error with variadiac template: "parameter pack must be expanded"

Question

Here's a variadic template function that I've written:

template<class Container, class Value, class... Args>
Value& insert(Container& c, Args&&... args) {
    c.emplace_back(args);
    return c.back();
}

When I use insert like this I'm getting an error:

list<int> lst;
int& num = insert<list<int>, int, int>(lst, 4);

The error complains about this line in the body of insert:

c.emplace_back(args); // <= 'args' : parameter pack must be
                      //             expanded in this context

What does that mean and how can I fix it?

`Value` should also be the first template argument so that template-argument deduction can be used for the other arguments (i.e `insert(lst, 4);`) — David G, Dec 14 '13 at 21:04
And you forgot to forward: `c.emplace_back(std::forward(args)...);` — dyp, Dec 14 '13 at 21:08
A container is required to have a member `value_type`, you can use it to get rid of the `Value` template parameter: `template typename Container::value_type& insert(Container& c, Args&&... args); insert(lst, 4);` — dyp, Dec 14 '13 at 21:10

score 38 · Accepted Answer · edited Sep 03 '17 at 19:05

38

The error is due to the missing ellipsis (...) after args when passing all individual parameters (rather than the parameter pack) to emplace_back.

The fixed (and improved) version:

template<class Container, class... Args>
auto insert(Container& c, Args&&... args) -> decltype (c.back()) {
    c.emplace_back(std::forward<Args>(args)...);
    return c.back();
}

edited Sep 03 '17 at 19:05

Florian Winter

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answered Dec 31 '13 at 00:42

Jarod42

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Error with variadiac template: "parameter pack must be expanded"

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