1 Introduction  
2 Formulation  
2.1 Problem Definition  
2.2 Multi-Directional Dimensionality Reduction  
2.3 Multi-Type Input Representations  
3 Macro-View: What Structure is Used to Fuse the CDSR Information?  
3.1 Multi-Level Cross-Domain Fusion Structures  
3.2 Bridges for Cross-Domain Fusion  
4 Micro-View: What Technologies are Used to Address the CDSR Problem?  
4.1 Basic Technologies  
4.2 Auxiliary Learning  
5 Datasets and Experimental Results  
6 Future Directions  
7 Conclusions