Unordered_map with custom class as key

» cpp » Unordered_map with custom class as key

By : user2186730

Date : November 25 2020, 04:01 AM

will be helpful for those in need I am working on a compiler right now and I am trying to represent the scope class by inheriting from unordered_map, because it is essentially a hash table of symbols to declarations. I added a custom hash function to the symbol, but I am getting an error complaining there is no default constructor for initializing a std::pair. Here is the relevant code:

code :

emplace(d->get_name()->get_str(), d);

/Library/Developer/CommandLineTools/usr/include/c++/v1/utility:422:5: note: candidate constructor not viable: no known conversion from
      'std::__1::basic_string<char>' to 'const const Symbol' for 1st argument
    pair(_T1 const& __t1, _T2 const& __t2)

Share :

insert custom class into unordered_map c++

By : Max Rahleev

Date : March 29 2020, 07:55 AM

wish of those help None of the two-parameter insert methods matches insert(key_type, mapped_type), which is what you are attempting.
The map holds std::pair, so you need to insert a pair, either explicitly:

code :

my_items.insert(std::make_pair(Node(33), 894));

my_items.insert({Node(33), 894});

my_items.emplace(Node(33), 894);

GNU C++: why custom class can not be value type for unordered_map?

By : vesaiosdev

Date : March 29 2020, 07:55 AM

may help you . This is most probably a stdlibc++ bug, in the version you're using.
I'm on ubuntu using gcc 4.8.1, and clang 3.5 trunk. With clang using libc++ there's no problem. With both clang and gcc using libstdc++ the issue is triggered.

code :

int main() {
    using namespace std;
    cout << is_convertible<const A &, A>::value << endl;
}

/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/type_traits:117:14: error: base class has incomplete type
    : public conditional<_B1::value, _B2, _B1>::type
      ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/bits/stl_pair.h:122:19: note: in instantiation of template class
      'std::__and_<std::is_convertible<const int &, const int>, std::is_convertible<const A &, A> >' requested here
               enable_if<__and_<is_convertible<const _U1&, _T1>,
                         ^
/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/bits/stl_pair.h:124:12: note: in instantiation of default argument for
      'pair<const int, A>' required here
        constexpr pair(const pair<_U1, _U2>& __p)
                  ^~~~
/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/bits/stl_pair.h:124:12: note: while substituting deduced template arguments into
      function template 'pair' [with _U1 = const int, _U2 = A, $2 = <no value>]
        constexpr pair(const pair<_U1, _U2>& __p)
                  ^
/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/type_traits:803:24: note: in instantiation of default argument for
      '__test<std::pair<const int, A>, const std::pair<const int, A> &>' required here
      static true_type __test(int);
                       ^~~~~~~~~~~
/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/type_traits:803:24: note: while substituting deduced template arguments into
      function template '__test' [with _Tp = std::pair<const int, A>, _Arg = const std::pair<const int, A> &, $2 = <no value>]
      static true_type __test(int);
                       ^
/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/type_traits:117:14: note: (skipping 10 contexts in backtrace; use
      -ftemplate-backtrace-limit=0 to see all)
    : public conditional<_B1::value, _B2, _B1>::type
             ^
/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/bits/unordered_map.h:97:27: note: in instantiation of template type alias
      '__check_copy_constructible' requested here
    class unordered_map : __check_copy_constructible<_Alloc>
                          ^
/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/type_traits:1305:42: note: in instantiation of template class
      'std::unordered_map<int, A, std::hash<int>, std::equal_to<int>, std::allocator<std::pair<const int, A> > >' requested here
        static decltype(__test_aux<_To1>(std::declval<_From1>()), __one())
                                         ^
/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/type_traits:1306:2: note: while substituting explicitly-specified template
      arguments into function template '__test'
        __test(int);
        ^
/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/type_traits:1319:11: note: in instantiation of template class
      'std::__is_convertible_helper<const A &, A, false>' requested here
                               __is_convertible_helper<_From, _To>::value>
                               ^
c++1y-sample.cpp:33:13: note: in instantiation of template class 'std::is_convertible<const A &, A>' requested here
    cout << is_convertible<const A &, A>::value << endl;
            ^
/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/type_traits:1317:12: note: definition of 'std::is_convertible<const A &, A>' is
      not complete until the closing '}'
    struct is_convertible

unordered_map of different custom types

By : awaymoko

Date : March 29 2020, 07:55 AM

it should still fix some issue Suppose if I have these two enums: , I believe you're looking for nested maps:

code :

std::unordered_map<Type, std::unordered_map<Location, unsigned int>>

unordered_map for custom class does not cause error when inserting the same key

By : nithin veerakumar

Date : March 29 2020, 07:55 AM

Hope this helps Your hash and operator == must satisfy a consistency requirement that they currently violate. When two objects are equal according to ==, their hash codes must be equal according to hash. In other words, while non-equal objects may have the same hash code, equal objects must have the same hash code:

code :

size_t operator()(const Line& k) const  {
    return hash<float>()(k.getM());
}

C++ unordered_map using a custom class type as the key

By : BoN

Date : March 29 2020, 07:55 AM

may help you . To be able to use std::unordered_map (or one of the other unordered associative containers) with a user-defined key-type, you need to define two things: