Difference between implicit conversion and explicit conversion [duplicate]

问题

Possible Duplicate:
Implicit VS Explicit Conversion

What is the difference between "implicit conversion" and "explicit conversion"? Is the difference different in Java and C++?

回答1:

An explicit conversion is where you use some syntax to tell the program to do a conversion. For example (in Java):

int i = 999999999;
byte b = (byte) i;  // The type cast causes an explicit conversion
b = i;              // Compilation error!!  No implicit conversion here.

An implicit conversion is where the conversion happens without any syntax. For example (in Java):

int i = 999999999;
float f = i;    // An implicit conversion is performed here

It should be noted that (in Java) conversions involving primitive types generally involve some change of representation, and that may result in loss of precision or loss of information. By contrast, conversions that involve reference types (only) don't change the fundamental representation.

Is the difference different in Java and C++?

I don't imagine so. Obviously the conversions available will be different, but the distinction between "implicit" and "explicit" will be the same. (Note: I'm not an expert on the C++ language ... but these words have a natural meaning in English and I can't imagine the C++ specifications use them in a contradictory sense.)

回答2:

You Mean Casting? Implicit mean you pass an instance of type, say B, that inherits from a type, say A as A.

For example:

Class A;
Class B extends A;

function f(A a) {...};

main() {
  B b = new B;
  f(b); // <-- b will be implicitly upcast to A.
}

There are actually other types of implicit castings - between primitives, using default constructors. You will have to be more specific with your question.

implicit with default constructor:

class A { 
  A (B b) { ... };
}

class B {};

main() {
  B b = new B();
  A a = b; // Implict conversion using the default constructor of A, C++ only.
}

回答3:

Casting is an explicit type conversion, specified in the code and subject to very few rules at compile time. Casts can be unsafe; they can fail at run-time or lose information.
Implicit conversion is a type conversion or a primitive data conversion performed by the compiler to comply with data promotion rules or to match the signature of a method. In Java, only safe implicit conversions are performed: upcasts and promotion.\

Also I would suggest reading about C++ implicit coversion: http://blogs.msdn.com/b/oldnewthing/archive/2006/05/24/605974.aspx

来源：https://stackoverflow.com/questions/7544233/difference-between-implicit-conversion-and-explicit-conversion

标签

java

c++

implicit-conversion

explicit-conversion