C++ auto& vs auto

Question

When creating local variables, is it correct to use (const) auto& or auto?

e.g.:

SomeClass object; const auto result = object.SomeMethod(); 

or const auto& result = object.SomeMethod();

Where SomeMethod() returns a non-primitive value - maybe another user-defined type. My understanding is that const auto& result is correct since the result returned by SomeMethod() would call the copy constructor for the returned type. Please correct me if I am wrong.

What about for primitive types? I assume const auto sum = 1 + 2; is correct.

Does this also apply to range based for loops?

for(const auto& object : objects) 

Answer 1

auto and auto && cover most of the cases:

• Use auto when you need a local copy. This will never produce a reference. The copy (or move) constructor must exist, but it might not get called, due to the copy elision optimization.

• Use auto && when you don't care if the object is local or not. Technically, this will always produce a reference, but if the initializer is a temporary (e.g., the function returns by value), it will behave essentially like your own local object.

  Also, auto && doesn't guarantee that the object will be modifiable, either. Given a const object or reference, it will deduce const. However, modifiability is often assumed, given the specific context.

auto & and auto const & are a little more specific:

• auto & guarantees that you are sharing the variable with something else. It is always a reference and never to a temporary.

• auto const & is like auto &&, but provides read-only access.

What about for primitive/non-primitive types?

There is no difference.

Does this also apply to range based for loops?

Yes. Applying the above principles,

• Use auto && for the ability to modify and discard values of the sequence within the loop. (That is, unless the container provides a read-only view, such as std::initializer_list, in which case it will be effectively an auto const &.)
• Use auto & to modify the values of the sequence in a meaningful way.
• Use auto const & for read-only access.
• Use auto to work with (modifiable) copies.

You also mention auto const with no reference. This works, but it's not very commonly used because there is seldom an advantage to read-only access to something that you already own.

Answer 2

Yes, it is correct to use auto and auto& for local variables. When getting the return type of a function, it is also correct to use auto&. This applies for range based for loops as well.

General rules for using auto are:

• Choose auto x when you want to work with copies.
• Choose auto &x when you want to work with original items and may modify them.
• Choose auto const &x when you want to work with original items and will not modify them.

You can read more about the auto specifier here.