Xcode learns C + + (third, const)

Const can limit the value of a variable not allowed to be changed, using the const to some extent can improve the security and reliability of the program

const int a = 10;

The value of variable a cannot be modified, it is always the initial 10

int const A = 10; is equivalent to the above notation

void sum (const int A, const int b)

The above function prevents others from tampering with the value of the parameter

As you can see, const can be used to define a constant. function with enum, #define类似

The following example shows that P is a constant and can no longer be assigned p, so p can not point to another variable

int a = 10;

int * Const P = &a;

The following example indicates that *p is a constant and cannot be modified by *p to modify the value of variable a

int a = 10;

const int *P = &a;

The following example prevents the parameter a pointer of the test function from modifying the value of the outside age variable

void Test (const int * a);

int age = 10;

Test (&age);

The following example shows that both *p and P are constants

int a = 10;

const INT * Const P = &a;

Const and # define

Both const and # define can be used to define constants, which can result in a change to all of them.

In fact, it is more recommended to use Const or enum to define constants

The initial purpose of the const rollout is to replace the precompiled directive, eliminate its drawbacks, and inherit its advantages

Const can save space and avoid unnecessary memory allocations. Example:

#define PI1 3.14159//Macro Constants

Const double PI2 = 3.14159; Pi is not placed in RAM at this time

Double A = PI2; Allocate memory for PI at this time, no longer assigned!

Double b = PI1; Macro substitution during compilation, allocating memory

Double C = PI2; No memory Allocations

Double d = PI1; Another macro replacement, another memory allocation!

Const definition constants from a compilation point of view, just give the corresponding memory address, instead of the immediate number as given in # define, so the const definition of the constant in the program run only one copy, and # define the constants in memory have several copies

#define的副作用

#define有时候会产生一些不好的副作用, such as the following example:

MAX macro returns the maximum value from two values

#define MAX (x, y) ((x) > (y)? (x): (y))

int a = 10;

int B = 6;

int c = MAX (++a, b);

cout << "c =" << C << ", a =" << a << Endl;

This time the output is:c =, a =

If the initial value of variable B is changed to 15, that is int b = 15;

The output of this time is:C = all, a = one

#define的特殊用途

#define还是有它无可取代的地方, like

Pass in a parameter, and then generate a string

#define TO_STR (x) #x

cout << to_str (ABCD) << Endl;

Concatenate multiple parameters to synthesize a complete identifier

#define CONCAT (x, y) x# #y

int myage = 20;

cout << CONCAT (My, age) << Endl;

Xcode learns C + + (third, const)