Question

Iterating over a vector works:

std::vector<int> collection = {2, 3, 4, 5435345, 2};
std::for_each(collection.begin(), collection.end(), [](int& i){cout << i << endl;});

but not over a set (compile error):

std::set<int> collection = {2, 3, 4, 5435345, 2};
std::for_each(collection.begin(), collection.end(), [](int& i){cout << i << endl;});

Why can’t I iterate over a std::set with std::for_each?

Bonus question:
Also, I would like to change the int& in the lambda’s argument to auto&, why can’t this be automatically deduced?

Answer 1

std::set<T>::value_type is T const, not T; consequently, the argument to your lambda must be a value type (i.e., copy) or int const& (and technically, or int const volatile&), not int&. I.e., this works:

std::set<int> collection{2, 3, 4, 5435345, 2};
std::for_each(
    collection.begin(),
    collection.end(),
    [](int const& i) { std::cout << i << std::endl; }
);

---

Bonus question: Also, I would like to change the int& in the lambda’s argument to auto&, why can’t this be automatically deduced?

Because the standard says it can’t; historically, I believe this was due to overly-complicated interactions between lambdas and concepts (before concepts were removed from the draft). However, I hear rumors that the first defect reports to the new (C++11) standard will address exactly this, so it’s possible that you’ll see support for this added to your compiler of choice within the next year or two. EDIT: Oh, look, C++14 now has polymorphic lambdas…