CODEWITHSUNDEEP
c++11pass-by-referenceperfect-forwardingforwarding-reference
justHelloWorld
justHelloWorld

Reputation: 6818

How to forward lvalue reference?

I have this code:

struct MultiMemoizator {
    template <typename ReturnType, typename... Args>
    ReturnType callFunction(std::function<ReturnType(Args...)> memFunc, const Args&... args) {
         return memFunc(args ...);
    }
};

int main()
{
    typedef vector<double> vecD;
    //filling vecD with random numbers...
    MultiMemoizator mem;
    function<vecD(vecD)> sort_vec = [](vecD &vec) {
        sort(vec.begin(),vec.end());
        return vec;
    };
    mem.callFunction<vecD,vecD>(sort_vec,vec);
    //vec is still not sorted!
}

Since memFunc(args ...); what happens is that a copy of args is sorted and not vec, so at the end vec will be unsorted after callFunction(...).

I think that in order to solve this problem forward can help me, but if I try: return cachedFunc(forward<Args>(args) ...); then something bad happens (like vector::size=0).

How can I forward args reference to sort_vec?

Upvotes: 0

Views: 49

Answers (1)

Andrei R.
Andrei R.

Reputation: 2462

specify return type as Type& or const Type& and return something that is not a temporary value. Like this:

function<vecD&(vecD&)> sort_vec = [](vecD &vec) -> vecD& {
    std::sort(vec.begin(),vec.end());
    return vec;
};

p.s. you vere trying to sort const vector.

Upvotes: 1

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