Push_back variadic function parameters into a vector?

Question

I am trying to push_back the parameters of a variadic function as shown below, but the compiler says there is a type mismatch (due to the parameters being a general type while the vector is int). What should I do to make the parameters compatible?

vector<int> x;
template<typename... Rest>
void foo(Rest... rest) {
    x.push_back(rest...);
}

score 8 · Accepted Answer · answered Apr 03 '17 at 23:12

8

In C++14 and before:

void foo(Rest... rest) {
    int a[] = {0, (x.push_back(rest), 0)...};
    static_cast<void>(a);  // unused
}

In C++17:

void foo(Rest... rest) {
    (x.push_back(rest), ...);
}

answered Apr 03 '17 at 23:12

Kerrek SB

464,522
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875
1,084

1

Thanks! What is the purpose of a[] though? – Ansel Chang Apr 03 '17 at 23:28
are you sure those push_backs has to be evaluated left to right? otherwise you might get items in confusing order. – sp2danny Apr 03 '17 at 23:56
@sp2danny: Yes. – Kerrek SB Apr 04 '17 at 15:58

score 1 · Answer 2 · answered Apr 03 '17 at 23:43

Start with a base case:

void push_all(vector<int>&) {}

Continue with 'gradual unpacking'

template<typename... Rest>
void push_all(vector<int>& vec, int val, Rest... rest)
{
    vec.push_back(val);
    push_all(vec, rest...);
}

Not as compact, but a little easier to grok.

Olli · Answer 3 · 2022-04-01T20:08:10.553

0

You should prefer this version as it's faster and works on C++11 too.

template<typename... Rest>
void foo(Rest... rest) {
    x.insert(x.end(), { static_cast<decltype(x)::value_type>(rest)...});
}

But if you for some reason need to use push_back() then at least call reserve() to make it faster.

template<typename... Rest>
void foo(Rest... rest) {
    x.reserve(sizeof...(Rest));
    (x.push_back(rest), ...);
}

edited Apr 01 '22 at 20:08

answered Apr 01 '22 at 17:26

Olli

29
5

This does not provide an answer to the question. Once you have sufficient [reputation](https://stackoverflow.com/help/whats-reputation) you will be able to [comment on any post](https://stackoverflow.com/help/privileges/comment); instead, [provide answers that don't require clarification from the asker](https://meta.stackexchange.com/questions/214173/why-do-i-need-50-reputation-to-comment-what-can-i-do-instead). - [From Review](/review/late-answers/31436106) – kamshi Apr 01 '22 at 19:19
@kamshi "This does not provide an answer to the question." Please explain... – Olli Apr 01 '22 at 19:51
op asked about how to make the parameters `Rest...` "compatible". What he probably means is how to store arbitrary types in a vector, for example using std::any. Your solution doesn't solve this problem, you only focused on performance (see here, it does not compile for non-integer types: https://godbolt.org/z/9dGPbnoGv), which is why I flagged your post as not answering the question. – kamshi Apr 01 '22 at 20:02
Sorry, I edited my answer to fix that. Thank you for noticing my mistake though. – Olli Apr 01 '22 at 20:11

Push_back variadic function parameters into a vector?

3 Answers3