How to use std::pair with classes? - c++

Question

so I'm experimenting with cvc5 and just wanted to keep track of the Terms in a map so I have created this:
std::map<std::string, std::pair<Term, int>> terms;
Basically, for I used the name as an index and I store the Term with other info in the map.
I have created a subtype of Term called TermStruct and I wanted to create another similar map:
std::map<std::string, std::pair<TermStruct, int>> termsStructs;
TermStruct was created roughly in the following way
class TermStruct : public Term {
public:
TermStruct(Term *t) : Term() {
this->t = t;
}
bool isNull();
Term *getTerm() { return this->t; };
std::string toString();
private:
Term *t = nullptr;
};
Now when I tried to add a new element to the termStructs map in the following way:
termsStructs[str] = std::pair(term, offset);
Note: term is of the correct type.
I have a number of compilation error such as:
/usr/include/c++/11/bits/stl_map.h:501:37: required from ‘std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type& std::map<_Key, _Tp, _Compare, _Alloc>::operator[](const key_type&) [with _Key = std::__cxx11::basic_string<char>; _Tp = std::pair<TermStruct, int>; _Compare = std::less<std::__cxx11::basic_string<char> >; _Alloc = std::allocator<std::pair<const std::__cxx11::basic_string<char>, std::pair<TermStruct, int> > >; std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type = std::pair<TermStruct, int>; std::map<_Key, _Tp, _Compare, _Alloc>::key_type = std::__cxx11::basic_string<char>]’
/home/alberto/progetti/llvm/plugin/runtime/cvc5/Runtime.cpp:113:25: required from here
/usr/include/c++/11/tuple:1824:9: error: no matching function for call to ‘std::pair<TermStruct, int>::pair()’
1824 | second(std::forward<_Args2>(std::get<_Indexes2>(__tuple2))...)
Any idea why and how to fix it?
Thanks

Answer 1

pair has nothing to do with this problem. It is all about map. I see two options.
Introduce a default constructor(a constructor without parameters) for TermStruct if you want to use std::map::operator[]. Here's why:
termsStructs[str] = std::pair(term, offset) does not insert the pair object right away
termsStructs[str] first creates a new key-value pair and adds it to the map(if there is no entry for str)
It is done by running std::make_pair(key, T())
As T here is std::pair, it tries to call Term() (FYI the second(integer) is zero-initialized)
However there is no Term() defined - compiler error
You may refer these links for details.
https://en.cppreference.com/w/cpp/container/map/operator_at (1)
https://en.cppreference.com/w/cpp/utility/pair/pair (1)
Or you can use emplace or insert.
These methods look uglier than the above but they behave just like what you must have expected. It does not require a default constructor.
For example,
termsStructs.emplace(str, std::pair(term, offset));
termsStructs.insert({str, std::pair(term, offset)});