Built-in debugger in Visual Studio has very nice extension capabilities. One particularly useful feature – developer can create custom visualizers for his own complex data types. Usually interactive debugger just shows data members of user-defined (and unknown to him) classes and structures, e.g.:
Obviously this is not very handy. In example above
mpreal is arbitrary precision floating-point numeric type. It is only natural to show variables of the type as numbers, not as collection of low-level data pointers and properties.