Expand std::vector into parameter pack

Question

I have methods with the following signature:

void DoStuff(int i);
void DoStuff(int i, k);
void DoStuff(int i, int k, int l);

I have a method from where I would like to call the DoStuff methods as follows:

void CallDoStuff(const std::vector<int>& vElements) {
  // What magic is supposed to happen here to make vElements an expandable pack?
  DoStuff(vElemets...);
}

Is there any chance to achieve this? Is using std::index_sequence the right way? If yes, could you please provide me a simple example how to apply this to my problem?

Answer 1

It's possible, as long as you provide an upper bound to the number of arguments.

Using Xeo's implementation of std::index_sequence for C++11:

template <unsigned... Idx>
void trampoline(const std::vector<int>& vElements, seq<Idx...>) {
    return DoStuff(vElements[Idx]...);
}

template <std::size_t Arity>
void trampoline(const std::vector<int>& vElements) {
    return trampoline(vElements, typename gen_seq<Arity>::seq{});
}

template <unsigned... Idx>
void CallDoStuff(const std::vector<int>& vElements, seq<Idx...>) {
    using trampoline_t = void (*)(const std::vector<int>&);
    constexpr trampoline_t trampolines[]{
        trampoline<Idx>...
    };
    trampolines[vElements.size()](vElements);
}

template <std::size_t Max>
void CallDoStuff(const std::vector<int>& vElements) {
    assert(vElements.size() <= Max);
    return CallDoStuff(vElements, typename gen_seq<Max + 1>::seq{});
}

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