fowarding reference | Together C & C++ | Page 1

crimson quiver Jul 20, 2023, 2:03 AM

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hi all, can anyone give me some examples where std::forward is useful? It seems the function will return lvalue or rvalue depending on what we passed in, but I dont see why we really need it, we can just pass it directly and resolve them through function overloading?

strange sirenBOT Jul 20, 2023, 2:03 AM

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trail willow Jul 20, 2023, 2:10 AM

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http://thbecker.net/articles/rvalue_references/section_01.html

nova robin Jul 20, 2023, 6:54 AM

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that is use for template functions. cuz a template class argument with"&&" can not only accept rvalue but also lvalue, so we need to use std::forward to return its original atrribution if we wanna call overload functions that distinguish by rvalue or lvalue, for example move constructor or copy constructor.

uncut oasis Jul 20, 2023, 8:27 AM

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crimson quiver hi all, can anyone give me some examples where std::forward is useful? It seems ...

the point is that you don't need function overloading with forwarding references, and it's not feasible to do it anyway

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for example std::make_unique<T> takes a pack of forwarding references which is passed to the constructor of T

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you can't really make an overload for every possible combination of lvalue and rvalue references due to the nature of this being a variadic function

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and you need std::forward for forwarding references because when accessed, they all turn into lvalues by default

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you wouldn't get any move semantics without it

grave ginkgo Jul 21, 2023, 12:56 PM

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https://stackoverflow.com/questions/3582001/what-are-the-main-purposes-of-stdforward-and-which-problems-does-it-solve
This is a great article on this topic

Stack Overflow

What are the main purposes of std::forward and which problems does ...

In perfect forwarding, std::forward is used to convert the named rvalue references t1 and t2 to unnamed rvalue references. What is the purpose of doing that? How would that affect the called function

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Basically it is used to solve the equivalence problem. If you have a function that calls another function, the inner function will see every parameter as lvalues. This causes problems when you are trying to make the inner function call a move constructor, for example, because you need rvalues to do that. std::forward works by maintaining the value category of a parameter so that problem doesn't happen.

crimson quiver Jul 21, 2023, 3:24 PM

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wait, so using function parameters inside the function will be treated as lvalues ?

uncut oasis Jul 21, 2023, 3:33 PM

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crimson quiver wait, so using function parameters inside the function will be treated as lvalue...

yes, even if they're rvalue references

trail willow Jul 21, 2023, 4:38 PM

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if it has a name, it's an lvalue

uncut oasis Jul 21, 2023, 4:46 PM

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