gistfile1.cpp

std::vector<std::function<Matrix5d()>> covariances;
for (int i(0); i<10000000; ++i) {
    // delay calculation by sending back a lambda
    //r3 += m*c*m.transpose();
     covariances.push_back([=]() {return m*c*m.transpose();});
}
for (auto fun: covariances) {
    if (rand.Rndm() > 0.75) {
        r3 += fun();                                                                                                                                                       
    }
}