Exception safety and make_unique

Question

Just to clarify, using make_unique only adds exception safety when you have multiple allocations in an expression, not just one, correct? For example

Answer 1

Not only when you have multiple allocations, but whenever you can throw at different places. Consider this:

f(make_unique(), function_that_can_throw());

Versus:

f(unique_ptr(new T), function_that_can_throw());

In the second case, the compiler is allowed to call (in order):

• new T
• function_that_can_throw()
• unique_ptr(...)

Obviously if function_that_can_throw actually throws then you leak. make_unique prevents this case.

And of course, a second allocation (as in your question) is just a special case of function_that_can_throw().

As a general rule of thumb, just use make_unique so that your code is consistent. It is always correct (read: exception-safe) when you need a unique_ptr, and it doesn't have any impact on performance, so there is no reason not to use it (while actually not using it introduces a lot of gotchas).