CODEWITHSUNDEEP
c++c++17
Suslik
Suslik

Reputation: 1071

What are the advantage of using move in c++

I have the following function

std::pair<Eigen::ArrayXXd, Eigen::VectorXd> f(...){
    ...
    auto a = Eigen::ArrayXXd(N,M);
    auto b = Eigen::VectorXd(M);
    ...
    return {std::move(a), std::move(b)};
}

int main() {
     ...
     const auto &[a_up, b_up] = f(...);
     writeToFile("b_up.txt", b_up);
     ...
}

In the Function f we allocate and initialize the Eigen Array and Vector a and b. With the move-keyword we still use the same memory that was allocated inside the function f. With const auto & b_up = f(...) also outside the function f the same memory space is used, so we never had to copy anything. Is this correct? But what is the advantage over Pass-by-reference. Does there copying happen?

Upvotes: 0

Views: 96

Answers (1)

Arpegius
Arpegius

Reputation: 5887

The return statement in f function creates std::pair witch is not copied further, due to RVO, even if you write:

auto ab_pair  = f(...);

But still the a and b have to be moved or copied, but there is a way to construct that pair in place using std::piecewise_construct and then return it by reference with RVO:

auto f(...){
    std::pair<Eigen::ArrayXXd, Eigen::VectorXd> result{
        std::piecewise_construct,
        std::make_tuple(N,M),
        std::make_tuple(M),
    };

    ...

    return result;
}

Upvotes: 2

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