Learning Note: constant

一. Java

Java use `final` to represent a constant:

1. compile-time constant
2. value initialized at run-time but not change ever since

data can be：

1. primitive -- the data can't be changed
2. object reference -- the reference can't change but the object can change

   // can also be compile-time constants:

    public static final int VALUE_THREE = 39;
    // can also be run-time constants:
    private final int i4 = rand.nextInt(20);
    static final int INT_5 = rand.nextInt(20);
    // a final reference
    private final String s1 = new String("s1");

 

二. c++

c++ using the keyword `const` to address this:

1. compile-time constant
2. value initialized at run-time but not change ever since (see following -- cited from stackoverflow)

As a static or function-local variable: const int x = calcConstant();     As a class member: struct ConstContainer { ConstContainer(int x) : x(x) {} const int x; };

data can be：

1. primitive and object -- the data can't be changed
2.  pointer --  you can choose pointer or the memory it point to
 

    const int *p; // can't change that int
    int * const p; // can't change pointer
    int const *p; // the same as first
    const int * const p; // can't change both

 

Something else:

If you want to use the  a constant array in separated file, you have to add `extern` before it for the const in c++ is default to be internal linkage, ie it is invisible outside of the file.

    // a.h: the size have to larger than number of elements in initializing list
    extern const int a[size];

   // a.cpp: size >= 2
    extern const int a[] = {1, 2 };

Reference:

1. Where to put compile-time-constant arrays?
   

2. C++ Assign a const value at run-time?

More:

1. Java's final vs. C++'s const

Gits:

1. Java constant – final
   
2. extern_const.cpp