const Class Type Parameters

For class types, there is a different general rule. If we take a class type parameter by value, we make a copy of it. In general, copying an object is more expensive than just passing a reference around.

So the rule to follow is not to take an object by value but by const&. Reference to avoid the copy and const so that the object doesn’t get modified.

We make local variables const by default similarly, we should take references and pointers const. If we want to modify the original object, we can take it by reference and omit the const.

On the contrary, if we want to modify only the object in a local scope, we can take it by value as we will have to make a copy of it.

Make a copy

Don’t take an object by const& if we are making a copy.

void doSomethingAndModifyLocally(const ClassA& foo) {
    // ...
    ClassA foo2 = foo;
    foo2.modify();
    // ...
 }

In the above case, we should take foo by value instead. We can spare the cost of passing around a reference and the mental cost of declaring another variable and calling the copy constructor.

Pass by value, if you need a local modification

If we modify an object only in a local scope, we should take it by value to make a copy of it.

void doSomethingAndModifyLocally(ClassA foo) {
    // ...
    foo.modify();
    // ...
 }