safecons.cpp

ThinkingC++中文版

//: C08:Safecons.cpp
// From Thinking in C++, 2nd Edition
// Available at http://www.BruceEckel.com
// (c) Bruce Eckel 2000
// Copyright notice in Copyright.txt
// Using const for safety
#include <iostream>
using namespace std;

const int i = 100;  // Typical constant	,i is a compile-time const
const int j = i + 10; // Value from const expr,j still is a compile-time const
long address = (long)&j; //requires the address of j and therefore forces the compiler to allocate storage for j. 
char buf[j + 10]; // Still a const expression

void main() {

  cout<<i<<endl;
  cout<<j<<endl;  //设置断点，察看i,j是否已分配存储？

  // It’s possible to use const for aggregates,so storage will be allocated. In these situations, const means “a piece of storage that cannot be changed.” 
  const int i[] = { 1, 2, 3, 4 };	
  float f[i[3]]; // Illegal	 //However, the value cannot be used at compile time because the compiler is not required to know the contents of the storage at compile time.
  struct S { int i, j; };
  const S s[] = { { 1, 2 }, { 3, 4 } };
   double d[s[1].j]; // Illegal

  // ...
} ///:~


/*
//file1
extern float j;	 //external linkage 

const int k=4;  //internal linkage 
//extern const int k=4;//explicitly define k as external linkage

void main()
{

	 float a = j*k;
}

float j=(float)2.2;	  //definition

//file2
extern float j;	 //	declaration

//extern const int k;

void fun() //definition
{
	j=10*k;
}
*/