Master the usage of const

stay C About language const Decorated variable ：

1.   C In language const Modified variables are called constant variables , It's not called a constant
2.   C A constant variable in a language is called a constant variable because it cannot be used as an lvalue , The rest are the same as variables , That is, the value of a constant variable can be modified
3.    Constant variables can be uninitialized , But if you don't initialize , Then there is no way to assign values directly , Assignment by pointer is required

Let's start with a piece of code ：

	const int a = 20;	// Constant variable 
	int* p = (int*)&a;
	*p = 30;
	//a It's a constant variable , Cannot be used as an lvalue , It's essentially a variable , So you can modify the value through the pointer 
	printf("%d %d %d\n", a, *p, *(&a));		// Output 30 30 30


	// Not initialized 
	const int b;
	int* pb = (int*)&b;
	*pb = 100;
	printf("%d %d\n", b, *pb);				// Output 100  100
	return 0;

  What do you think result of the this piece of the code will be ？？

Running results ：

Let's see C++ in const Decorated variable

1.  C++ in const The variable decorated is called a constant , Must be initialized , Cannot be used as an lvalue
2.  C++ in const Compilation method and C Different language ,C In language const Make an appropriate variable to compile the generated instruction ;C++ All appear in const Where constant names are , Are replaced by the initialization of constants     test01();
3.  C++ in const Decorated variables are called constants , But its value can still be modified through the pointer  

Let's look at this code ：

void test01()
{
	const int a = 20;	// Constant 
	int* p = (int*)&a;
	*p = 30;

	cout << a << " " << *p << " " << *(&a) << endl;		// Output 20 30 20
}

  What will be output after running ？？？

Output 20  30  20, When compiling this code , All that appears in the code const The modified variable name will be directly replaced with the initial value , That is to say 20. Using pointer pairs a Change the value of ,a The value of has been changed to 30, Through the dereference of the pointer, you can find that the value of the memory area has been changed to 30;

Let's take another look

Use a variable to initialize const Decorated variable , So this variable is no longer a constant , It's a constant

Code segment

void test02()
{
	int a = 20;
	const int b = a;	// Constant variable 

	int* p = (int*)&b;
	*p = 30;
	cout << b << " " << *p << " " << *(&b) << endl;
}

  The running result must be and C Language is the same .