[SOLVED] passing shared pointer in C++

Jerry Mcguire · 10-08-2018, 11:08 PM

When writing a function to accept a shared pointer or unique pointer in C++, should I make the pointer pass by value or pass by reference?

Code:

auto sp = std::make_shared<Object>();
target_func( sp );

Code:

void target_func( std::shared_ptr<Object>& p ); // or
void target_func( std::shared_ptr<Object> p ); // which one is the norm?

Thanks

pan64 · 10-09-2018, 01:59 AM

As far as I know: there is no general norm for this. You can do this way or that way, it depends on you.
In general both can have pros and cons.

mina86 · 10-09-2018, 04:11 AM

Passing by reference avoids reference counter increment so it may be faster (especially if you do it a lot in a highly multithreaded application) though the code may be considered a bit more confusing. Especially since, if you pass by reference, the method can change the shared pointer which could be a surprising behaviour; in this case you probably would want to a const reference. Then again, passing by reference stacks pointers on top of pointers so dereference could potentially be slower. Performance is hard to reason about in abstract.

As an aside, if you want to give away ownership of the object and pass it to the method, std::move the value and use &&.

dugan · 10-09-2018, 07:44 PM

https://stackoverflow.com/a/8741626/240515
https://herbsutter.com/2013/06/05/go...er-parameters/

Jerry Mcguire · 10-10-2018, 08:13 PM

Thank you all.

I personally prefer passing by reference over by value, because the shared_ptr is not a simple type after all.