Understanding a function return

Question

I am a novice programmer and have only briefly covered the anatomy of a function call (setting up the stack, etc.). I can write a function two different ways and I'm wondering which (if either) is more efficient. This is for a finite element program so this function could be called several thousand times. It is using the linear algebra library Aramdillo.

First way:

void Q4::stiffness(mat &stiff) 
{
    stiff.zeros; // sets all elements of the matrix to zero
    // a bunch of linear algebra calculations
    // ...
    stiff *= h;
}

int main()
{
    mat elementStiffness(Q4__DOF, Q4__DOF);
    mat globalStiffness(totalDOF, totalDOF);
    for (int i = 0; i < reallyHugeNumber; i++)
    {
        elements[i].stiffness(&elementStiffness, PSTRESS);
        assemble(&globalStiffness, &elementStiffness);
    }
    return 0;
}

Second way:

mat Q4::stiffness() 
{
    mat stiff(Q4__DOF, Q4__DOF); // initializes element stiffness matrix
    // a bunch of linear algebra calculations
    // ...
    return stiff *= h;
}

int main()
{
    mat elementStiffness(Q4__DOF, Q4__DOF);
    mat globalStiffness(totalDOF, totalDOF);
    for (int i = 0; i < reallyHugeNumber; i++)
    {
        elementStiffness = elements[i].stiffness(PSTRESS);
        assemble(&globalStiffness, &elementStiffness);
    }
    return 0;
}

I think what I'm asking is: using the second way is mat stiff pushed to the stack and then copied into elementStiffness? Because I imagine the matrix being pushed to the stack and then being copied is much more expensive than passing a matrix be reference and setting its elements to zero.

Answer 1

Passing a variable by reference and doing your calculations on that variable is a lot cheaper. When c++ returns a variable, it pretty much copies it twice.

First inside the function, and then it calls the copy constructor or assignment operator, depending on if the value is being assigned to a new variable or to an existing variable, to initialize the variable. If you have a user-defined variable with a long list of internal state variables then this assignment operation is going to take a big chunk of the operator's processing time.

EDIT#1: I forgot about c++11 and the std::move. Many compilers can optimize functions like this so they can use std::move and instead of copying an lvaue it can copy an rvalue which is just the memory location.