How to declare the return of `emplace()` of an instantiated `std::set`?

1. I have a class template:

   template<class idxT>
   class SimpleGraph {
   private:
     // ...
     std::set<std::pair<idxT,idxT>> edgeSet;
     // ...
     void addEdge(idxT a, idxT b);
     // ...
   }
   
   

2. I need to use the return of edgeSet.emplace(a,b) inside addEdge(), which is denoted as r in the following.

3. The declaration and the assignment of r need to be separated since the value of a and b is determined by a if-else statement. Thus, I cannot use

   auto r = edgeSet.emplace(a,b);
   
   

4. I tried the following strategies but neither is successful. The first one is

   std::pair<decltype(edgeSet)::iterator,bool> r;
   // some code
   r = edgeSet.emplace(a,b);
   
   

   The second one is

   // inside class template
   typedef std::set<std::pair<idxT,idxT>> EdgeSetType;
   EdgeSetType edgeSet;
   
   // inside definition of `addEdge()`
   std::pair<EdgeSetType::iterator,bool> r;
   // some code
   r = edgeSet.emplace(a,b);
   
   

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How can I declare the return of edgeSet.emplace(a,b)?