Inheritance from STL priority_queue with custom comparator not working

October 17, 2022

I would like to inherit from STL priority queue to have some additional functionality such as:
allowing removal. But I am struggling to make this work when I use custom comparators. MWE:

#include <queue>

template<typename T, class Container=std::vector<T>, class Compare=std::less<typename Container::value_type>> 
class custom_priority_queue : public std::priority_queue<T, Container, Compare>
{
    public:
    // My additional functions here.
};

int main()
{
    auto pq_comp = [](const int& a, const int& b) { return a <= b; };
    std::priority_queue<int, std::vector<int>, decltype(pq_comp)> pq(pq_comp); // works
    custom_priority_queue<int> pq_custom; // works
    custom_priority_queue<int, std::vector<int>, decltype(pq_comp)> pq_custom2(pq_comp); // Error
    return 0;
}

The error is:

main.cpp: In function ‘int main()’:
main.cpp:15:87: error: no matching function for call to ‘custom_priority_queue, main():: >::custom_priority_queue(main()::&)’
   15 |     custom_priority_queue<int, std::vector<int>, decltype(pq_comp)> pq_custom2(pq_comp); // Error
      |                                                                                       ^
main.cpp:4:7: note: candidate: ‘custom_priority_queue, main():: >::custom_priority_queue(const custom_priority_queue, main():: >&)’
    4 | class custom_priority_queue : public std::priority_queue<T, Container, Compare>
      |       ^~~~~~~~~~~~~~~~~~~~~
main.cpp:4:7: note:   no known conversion for argument 1 from ‘main()::’ to ‘const custom_priority_queue, main():: >&’
main.cpp:4:7: note: candidate: ‘custom_priority_queue, main():: >::custom_priority_queue(custom_priority_queue, main():: >&&)’
main.cpp:4:7: note:   no known conversion for argument 1 from ‘main()::’ to ‘custom_priority_queue, main():: >&&’

>Solution :

Constructors are not automatically inherited, so your class probably lacks any constructor, except the implicitly-declared ones.

You can explicitly inherit all constructors of the base class:

template<typename T, class Container=std::vector<T>, class Compare=std::less<typename Container::value_type>> 
class custom_priority_queue : public std::priority_queue<T, Container, Compare>
{
    // inherit constructors
    using priority_queue::priority_queue;

    public:
    // My additional functions here.
};