4. Const and difine and the problem of initializing arrays with const and define

1.const Constant

const Is a constant variable , Decorated variables cannot be changed .

 const int num = 1;
 // num = 2;
 printf("num=%d", num);

If you will num=2 Remove comments from , That is to modify const Value , You're going to report a mistake .

const int num = 1;
num = 2;
printf("num=%d\n", num);

At this time, we associate the definition of array ,int arr[] = ;

We know that variables cannot be placed in brackets when defining an array , but const The defined variable is an unchangeable variable , Can we put const Defined variables ？

The answer is C Not in the file , stay Cpp Medium can .

    const int n = 10;
    int arr[n] = {1, 2, 3};
    for (int i = 0; i <= 9; i++)
    {
        printf("%d", arr[i]);
    }
    return 0;

This code is written in C In file

Written in Cpp In file  

  Why does it appear in this problem ？

         First, we need to understand what constants are , What is a read-only variable ？

• Constant It must be read-only , For example, digital 6, character string “abc” etc. , It must be read-only , because There is no place in the program to store its value , Of course, we can't modify it .
• A read-only variable It is in Open a place in memory to store its value , Only this value is limited by the compiler Not allowed to be modified .
• C Language Subscripts must be constants when defining arrays , Read only variables are not allowed .

therefore

        stay C In language ,const It is a modifier used to limit that a variable is not allowed to be changed , Read only variable , Because it occupies storage space , So the compiler doesn't know the value at run time , So I don't know how big the array should be defined .
        stay C++ in , const The modified can be regarded as a constant at compile time
For basic data types ： The compiler will put it in the symbol table , Without allocating storage space .
about ADT（Abstract Data Type Abstract data types ）/ UDT（ User defined type ） Of const Objects need to allocate storage space （ Big object ）.
And in some cases, it is also necessary to allocate storage space , For example, the mandatory declaration is extern Or get the address of the Symbolic Constant .

So how to C As in the C++ Then you can define the array ？ Let's see define

2.define

define Used to define identifier constants , It's also called hongdefination , The program will use define Defined content Replace

for example define N 100, Will be 100 To replace all N, In other words , differ const Constant ,define In fact, it is essentially the replacement of text content .

So in C You can use define To initialize an array

#define MAX 10
int main(int argc, char const *argv[])
{
    int arr[MAX] = {0};
    printf("%d", MAX);
    return 0;
}

It won't make a mistake .

Sum up , Let's see const and define The difference between

1.define Is a macro definition , The program will use define Replace the defined content . So when the program is running , There's no use in the constant table define Defined constant , The system does not allocate memory for it . and const Defined constant , When the program is running , Exists in the constant table , And The system allocates memory for it .
2.define Defined constant , The preprocessing is just a direct replacement , Therefore, data type verification cannot be performed at compile time .
and const Defined constant , stay Strict type checking at compile time , You can avoid mistakes .

tips：

We are choosing define still const When initializing the array , If it says C Language , Then use define, And in the c++ It is more recommended to use const, because define In the pretreatment stage Lack of type detection mechanism , Therefore, the type cannot be specified correctly , It will lead to some mistakes . and define Preprocessing macros are global . So in C++ So much emphasis on namespaces 、 In the language of such things , The best thing for the overall situation is to have as few things as possible .