"C + + Learning notes" Const usage __c++

C + + const allows you to specify a semantic constraint that the compiler enforces to allow the programmer to tell the compiler that a value is unchanged. If you do have a value in your programming that remains unchanged, you should explicitly use the const to get help from the compiler. 1.const modifier member Variable

A const can be used to modify a member variable to tell the compiler that the member variable remains unchanged. Particular attention should be paid to the const modifier pointer variable:

1 There is only one const, if the const is on the left of *, the data pointed to by the pointer is a constant and cannot be modified by the dereference; The pointer itself is a variable and can point to other memory cells.

2 There is only one const, if the const is on the right side, the pointer itself is a constant, cannot point to another memory address, the data that the pointer refers to can be modified by the dereference.

3) Two const,* or so, then the data that the pointer and the pointer cannot modify.

Examples are as follows:

int main ()
{
	int a1=3;  Non-const data
	const int A2=A1;	Const data
	int *a3=&a1;		Non-const data,non-const pointer
	const INT *a4=&a1;	Const DATA,NON-CONST Pointer
	int * CONST a5=&a1;	Non-const data,const pointer
	const INT * Const a6=&a1;		Const DATA,CONST Pointer
	const INT * Const a7=&a1;		Const DATA,CONST pointer return
	0;
}

2.const modifier function Parameters

The passed arguments cannot be changed within the function as they were when the variable was modified.

For example:

void Testmodifyconst (const int _x)
{
	_x=5;
}

When you compile the above function, you will get an error. Because the _x is immutable. 3.const decorated member functions

1 A const-decorated member function cannot modify any member variable except for mutable-modified variables.

2 A const member function cannot invoke a non-const member function because a non-const member function might modify a member variable.

For example:

#include <iostream>
using namespace std;
Class point{public
: Point
(int _x): X (_x) {}
void testconstfunction (int _x) const{///
error, in const member function , you cannot modify any class member variable
x=_x;
Error, the const member function cannot invoke a ONST member function because a non-const member function can modify a member variable.
modify_x (_x);
}
void modify_x (int _x) {
   	x=_x;
}
int x;
};

4.const decorated member functions

1) pointer passing

If you return the const Data,non-constpointer, the return value must also be assigned to the const data,non-const pointer. Because the data that the pointer points to is a constant that cannot be modified.

For example:

const int * MALLOCA ()	///const data,non-const pointer
{
	int *a=new int (2);
	return A;
}
int main ()
{
	const int *a=malloca ();
	int *b=malloca ();    Compilation error return
	0;
}

2) Value Delivery

If the function return value takes a "value pass", the const modifier has no value because the function copies the return value to the external temporary storage cell. So, for value transfer, adding a const doesn't mean much. 5. Examples of pen questions

1) 2014 Perfect World Choice question 10th question

The print () function is a regular member function of a class that has no return value, and the following representation is correct: A

void print () const b.constvoid print () c.void constprint () d.void print (const)

Parsing: A const is used to modify the member function, B is used to modify the function return value, c No such use, D is the use of the modifier function passed the parameter value.

2) 2013 Perfect World Choice question 6th question

struct Thing

{

int Valuea;

const int VALUEB;

};

Thing T;

The values of the member variables Valuea and VALUEB of T are respectively: C
A, 0 0 B, garbage value 0 C, unable to run D, garbage value garbage value

Parsing: Valueb is a const type and must be initialized before it can be done.