Smart cities typically encompass several of distributed systems engaged in complex relationships including the integration and interaction with other systems towards providing new functionalities. These relationships lead to the so-called systems-of-systems (SoS), a widespread set of independent, heterogeneous complex systems that have their own purpose and collaborate with each other to fulfill common global missions. The SoS concept has indeed emerged as an active domain of research in recent years and it is of crucial importance for the societal challenges facing the world. In this perspective, we herein present how a smart city itself can be holistically viewed as a complex SoS and shed light on some challenges related to the development of this type of systems in the context of smart cities.