Difference between lambda and member function pointer

Question

In my answer here, Barry pointed out that it's better to call views::transform(&Planter::getPlants) because views::transform([](Planter const& planter){... accidentally copies.

#if 1
    auto plants = planters
        | std::views::transform([](Planter const& planter){ return planter.getPlants();})
        | std::views::join
        | std::views::common
        ;
// Plant copy constructor
// Plant copy constructor
// Plant copy constructor
// Plant copy constructor
// Plant copy constructor
#else
    auto plants = planters
        | std::views::transform(&Planter::getPlants)
        | std::views::join
        ;
#endif
// Plant copy constructor
// Plant copy constructor

Here Plant is a wrapper around int and Planter is a wrapper around std::vector<int>.

https://godbolt.org/z/dr7PM5Tvd

score 7 · Accepted Answer · answered Oct 16 '22 at 02:30

7

Oh I actually know this one. The deduced return type of the lambda actually decays the const ref qualifiers of getPlants.

You can fix this by declaring the return type of the lambda to be decltype(auto)

views::transform([](Planter const& planter) -> decltype(auto){...});

https://godbolt.org/z/ocK5PG1z1

answered Oct 16 '22 at 02:30

Tom Huntington

2,260
10
20

Then how it is possible to be a copy? – UserUsing Oct 16 '22 at 04:56
@UserUsing you'll have to clarify – Tom Huntington Oct 16 '22 at 05:10
1

Good one, I lost count of how many times I've made this mistake. – thedemons Oct 16 '22 at 07:35
*views::transform([](Planter const& planter){... accidentally copies.* I am wondering how; because what we returning in the lambda is a ref wrapper to the vector of "Plant"/ int. This is a const ref, so no copy, right? – UserUsing Oct 16 '22 at 16:22
@UserUsing no the const ref decays for the lambda's deduced return type unless you have `-> decltype(auto)`. – Tom Huntington Oct 16 '22 at 16:27
That means const ref decays to just copy unless we say decltype (auto), right? – UserUsing Oct 16 '22 at 17:36
2

@UserUsing Yes, the return type of `getPlants` is `Planter const&` but without `decltype(auto)` the return type of the lambda is just `Planter` – Tom Huntington Oct 16 '22 at 21:24

Difference between lambda and member function pointer

1 Answers1

Linked