How to get the first element in a std::set of pointers, where the pointer points to a specific value?

Question

Say you have a C++ std::set of unique pointers e.g.

auto my_set = std::set<std::unique_ptr<std::string>> 
{ 
    std::make_unique<std::string>("monkey"),
    std::make_unique<std::string>("banana"),
    std::make_unique<std::string>("orange") 
};

Using the std::find_if function, how would you go about finding the first element within this set such that the pointer points to "orange"?

So far I have got:

auto food = "orange";
auto find_orange
   = std::find_if(my_set.begin(), my_set.end(), [&food](const auto& ptr) -> bool {
        return *(*ptr) == food;
    }
);

but this does not compile. Any ideas on what the issue is?

Also in these kinds of functions what exactly is ptr i.e. the parameter in the predicate? Is it a pointer to each element of the container?

Does it fail to compile because you've forgot a semicolon after "orange" on the first line? — bipll, Aug 03 '20 at 15:31
@eerorika it's part of a specific thing I need to do, I know it's not the most usual thing — user11508332, Aug 03 '20 at 15:32
Isn't `ptr` already a `const std::unique_ptr &`? It only needs 1 indirection. — François Andrieux, Aug 03 '20 at 15:32
`ptr` is `const std::unique_ptr &`, `*ptr` is `const std::string &`, and `*(*ptr)` would dereference a string, typo? — Lukas-T, Aug 03 '20 at 15:34
Can you even make a `std::set>` like this: https://godbolt.org/z/3redfW ?? — JeJo, Aug 03 '20 at 15:37
@JeJo [You can't](https://stackoverflow.com/questions/46737054/vectorunique-ptra-using-initialization-list). — François Andrieux, Aug 03 '20 at 15:43

score 2 · Accepted Answer · answered Aug 03 '20 at 15:43

The question lacks a proper minimal reproducible example. When we make one MCVC: https://godbolt.org/z/bfYx39

The problem is coming from lambda return

return *(*ptr) == food;

as the auto there is std::unique_ptr<std::string>. Therefore you need to just dereference once:

auto find_orange = std::find_if(my_set.begin(), my_set.end(),
      [&food](const auto& ptr) {
         return *ptr == food;
         //     ^^^^^^^^^^^^
      }
   );

you do not need to double dereferencing there.

How to get the first element in a std::set of pointers, where the pointer points to a specific value?

1 Answers1