可变参数模板功能和参考类型

Question

As part of a DSL for an OpenCL binding I have to count the arguments given to a function f and handle them seperately, in the code below this would be h . The function should accept LValues as well as RValues for most types. However, for some types (here: int ) only LValue references are acceptable (in the real code these are objects that might need to initialised lazily).

My problem: how do I get rid of that const_cast ? f(T...) doesn't work, neither does f(T&...) or f(T&&...)

template<typename T>
void h(int i, const T &x) {/* generic things */}

void h(int i, const int &x) { const_cast<int&>(x) = 123; }

template<int i> void g() {}

template<int i, typename H, typename... T>
void g(H &x, T... xs) {
    h(i, x);
    g<i + 1>(xs...);
}

template<typename... T>
void f(const T&... xs) { g<0u>(xs...); }

#include <cassert>
int main(int, char**) {
    int x = 1;
    f(x, 2.0 + 3.0, 'c');
    assert(x == 123);
}

Answer 1

While writing the question I found a solution, but I don't understand why the signature is int && instead of int & (doesn't seem to matter though?)

template<typename T>
void h(int i, T x) {cerr << i << "=" << x << endl;}

void h(int i, int &&x) {
    x = 123;
    cerr << i << "=" << x << " (new)" << endl;
}

template<int i> void g() {}

template<int i, typename H, typename... T>
void g(H &&x, T &&... xs) {
    h(i, std::move(x));
    g<i + 1>(xs...);
}

template<typename... T>
void f(T &&... xs) { g<0u>(xs...); }

Also, with h(int, T&) the overload h(int, int&&) is called for all three arguments, int , double and char , which looks very suspicious to me (or is this because int x = 2.0, y = 'a'; works, too?)