Unable to create unordered_set with lambda function

Question

I get the error

error: call to implicitly-deleted default constructor of '__compressed_pair_elem<(lambda at 
main.cpp:181:16), 1>': _Base1(std::forward<_Tp>(__t)), _Base2() {}

with the following code. What's the mistake I am making and I couldn't understand the error as well.

using namespace std;

auto my_hash = [](vector<int> const& vec){
    size_t seed = vec.size();
    for(auto& i : vec) {
        seed ^= i + 0x9e3779b9 + (seed << 6) + (seed >> 2);
    }
    return seed;
};

using MySet = unordered_set<vector<int>, decltype(my_hash)>;

int main() {
    vector<int> a{1,2,3};
    MySet s;
    return 0;
}

Vittorio Romeo · Accepted Answer · 2018-10-29 12:32:15Z

28

Closure types are not default-constructible prior to C++20 (see P0624). You need to pass in an instance of your hasher:

constexpr std::size_t bucket_count = 512;
MySet s{bucket_count, my_hash};

live example on wandbox.org

Alternatively, you can use a good old struct:

struct Hasher {
auto operator()(vector<int> const& vec) const {
    size_t seed = vec.size();
    for(auto& i : vec) {
        seed ^= i + 0x9e3779b9 + (seed << 6) + (seed >> 2);
    }
    return seed;
}
};

using MySet = unordered_set<vector<int>, Hasher>;

int main() {
    MySet s;
}

live example on wandbox.org

edited Oct 29, 2018 at 12:32

answered Oct 29, 2018 at 12:29

Vittorio Romeo

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5 Comments

Yakk - Adam Nevraumont Over a year ago

I am vaguely sad that that change also doesn't make trivial copyable capture lambdas trivial copyable in general.

user3285099 Over a year ago

When all can we not plugin lambda functions as functors directly? I remember we can plugin lambdas directly sometimes(like sort). @vittorio-romeo

Vittorio Romeo Over a year ago

@user3285099: when the type of the closure can be deduced

user3285099 Over a year ago

What does closure being deduced mean? I know close in the mathematics but couldn't understand in this context.

Vittorio Romeo Over a year ago

[]{} is a lambda expression, which produces a closure (object with operator()). The type of the closure is unique and anonymous, so you have to use decltype to retrieve it. In contexts such as function templates, the type can be deduced from the call site, so decltype is not required.

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