Java的演员和C ++的演员阵容

Question

I come from C++ and just know some Java, but there is one thing that I don't fully understand about java is the casting. In c++, we have Regular cast vs. static_cast vs. dynamic_cast . In java, we have casting between primitive types and object types (known as boxing and unboxing), casting between primitive types, casting using Class function, and sometimes we perform unchecked casting. Is there a direct mapping from Java's casting system to C++'s casting system? are they equivalent? or is there something that is unique in one side?

Answer 1

If you know C++, the Java object model can be explained fairly easily. Java variables of class type are references to dynamically created objects of the corresponding type. An initialized Java variable T x = new T(); corresponds closely to a C++ pointer: T * px = new T;

In Java, all class types inherit from the polymorphic type Object , and thus all pointers can be used for RTTI: All casts of Java class-type variables are what dynamic_cast is in C++. So Java's (S) x corresponds to dynamic_cast<S *>(px) in C++, with the same failure mode that a null result indicates that the dynamic types aren't related.

All the other C++ pointer casts are unsafe and have no equivalent in Java. I don't know if Java has scalar conversions like integer-to-floating point; if so, then those would correspond closely to the C++ conversions.

Boxing and primitive types are a rather separate and unrelated issue. If you wanted something like that in C++ you'd have some kind of template <typename T> struct Box with a conversion-to- T operator.

Answer 2

In short:

Think about casts from built-in types as static_cast's and

think about casts between objects as dynamic_cast's in C++.