Pitfalls & gotchas

Pitfalls & Gotchas

Given that Fastor is an expression template math library there are certain idioms and rules you have to follow while writing code that uses Fastor. In particular

The use of auto keyword

C++11 and more specifically C++14 allows the use of auto return keyword in functions. Consider

auto add() {
    Tensor<double,3,3> a,b;
    return a+b;
}

While there seems nothing incorrect in this function, you have to keep in mind that Fastor's expressions are not evaluated immediately in to a tensor. In the above example the addition between two tensors a+b does not result in a tensor. It results in an "addition expression" that holds references to tensors a and b. As a result the return type of this function is deduced to be an expression owing to the use the keyword auto and given that both a and b are local to this function the references contained in this addition expression will be dangling once you exit the scope of the function. So calling this function will not give the expected result

Tensor<double,3,3> c = add();

To fix this you can either change the return type of the function from auto to an appropriate tensor

Tensor<double,3,3> add() {
    Tensor<double,3,3> a,b;
    return a+b;
}

or if you must use the keyword auto then you can call the evaluate function on the expression on return

auto add() {
    Tensor<double,3,3> a,b;
    return evaluate(a+b);
}

Both these strategies will give the expected result.