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c++c++11c++14rvalue-reference

Difference between capturing return value by `const auto&` and `auto`

I've seen code like const auto& now = time(nullptr);

I think the purpose of assigning the rvalue returned by time is to extend the return value's lifetime, as explained in the GotW post. But why not use const auto now = time(nullptr)?

Why do we want to extend the lifetime of the return value here? and is there any performance difference?

Solution

  • In the shown example, there is no reason to use a reference. The behavior of the snippet is identical with or without it.

    In general, it's a good idea to use a reference to avoid incurring needless copies, so there's no harm in getting into the habit of using references, even if it doesn't make a difference in some cases.

    I'd suggest writing const on the east though. This is just a preference, and definitely doesn't change the meaning of the code, but it's more consistent. I would write it as:

    auto const & now = time(nullptr);