S>v_map.try_emplace(true, std::in_place_type<some_type>); // Жалко, что так нельзя!
S>

class Outer
{
public:
  
    Outer(int i, double d, const std::string& s)
        : m_inner(std::make_unique<Inner>(i, d, s)) { }

private:
    struct Inner
    {
        int i{};
        double d{};
        std::string s{};
   };

   std::unique_ptr<Inner> m_inner;
};


void foo()
{
    std::map<int, Outer> m;
    
    m.emplace(
        std::piecewise_construct,
        std::forward_as_tuple(1),
        std::forward_as_tuple(42, 3.14, "Hello")
    );
}