Re: Lifetime of a temporary passed in operator[](const std::string& k)

Is this code safe?

class A
{
public:
     class Proxy
    {
    public:
    friend class A;

         operator A&()
         {
             return val_.get(k_);
         }

         operator const A&() const
         {
             return val_.get(k_);
         }
    private:
        Proxy& operator = (const Proxy& other) {/* do nothing */}
        Proxy(const Proxy& proxy)
            : val_(proxy.val_), k_(proxy.k_)
        {
        }
        Proxy(A& val, const std::string& k)
            : val_(val), k_(k)
        {
        }

        A& val_;
        const std::string& k_;
    };

    A& get(const std::string& k)
    {
        // In real code, looks up another A instance using k
        return *this;
    }

    const A& get(const std::string& k) const
    {
        // In real code, looks up another A instance using k
        return *this;
    }

     Proxy operator[](const std::string& k)
     {
         return Proxy(*this,k);
     }
};

     A a;
     A b = a["key"];

Do I have to worry about the lifetime of the temporary k_ held in Proxy?