C++ When does it make sense to pass a const struct parameter by value vs. reference?

Question

I've seen a similar question to this, but i'd like some clarification...

Assuming a basic C++ class:

class MyClass
{
public:
    struct SomeData
    {
        std::wstring name;
        std::vector<int> someValues;
    };

    void DoSomething(const SomeData data);
}

I understand that data will be passed as const to DoSomething and that is ok since data will not be modified in any way by the function...but I am used to seeing & specified with const parameters to ensure that they are passed by reference, e.g.

void DoSomething(const SomeData& data);

That seems more efficient to me. If we omit the &, then isn't data being passed by value to DoSomething? I'm not sure why it would ever be preferable to pass a const parameter by value when you can pass by reference and avoid the copy occurring?

Answer 1

Pass by value/reference and const-correctness are two different concepts. But used together.

Pass by Value

 void DoSomething (SomeData data);

Pass by value is used when it is less costly to copy and do not want to keep references to foreign objects. This function could (if it is inside a class) keep a pointer to this in some case and have its own copy.

Pass by reference

void DoSomething (SomeData& data);

Always use pass by reference if you know this might cause a performance loss copying the struct. This function could (if it is inside a class) keep a pointer to this in some case and pointing to a foreign object. Keeping pointers to foreign objects mean you should aware of its life-time and when this foreign object goes out of bound. More importantly changes to foreign object appears to your pointer.

const correctness

void DoSomething (const SomeData data);  // const for local copy
void DoSomething (const SomeData& data); //  const for callers object

Adding constto pass by value or reference means this function does not change it. But not having or having & decides which object you are trying to add safety of modifying. const is a very helpful tool in C++ in terms of Documenting APIs, provide compile time safety, allow more compiler optimizations.

Read this article.