The basic function and usage of the Const keyword in c language

You see the const keyword, most people will not think it is a const constant, I tell you that this is wrong cognition, then what is the actual meaning of it? In fact, a const cannot change a variable to a constant, but rather a variable into a read-only variable, so we cannot modify its value directly, but we can modify its value by its address.

1. General usage of const

const int n = 5;

int const n = 5; The effect of these two sentences is the same, it is declared a read-only variable named a, we can not directly modify the value of a.

int A[n] = {1, 2, 3, 4, 5}; The size of the required array is a constant, that is, a definite value, and n is a read-only variable, which means that there is a definite value when n is initialized, so it can be used as the size of the array

2. Const for pointers

int a = 5;

int B = 10;

const INT * p = &a;

int const * p = &a; The function of these two sentences is the same, it is declared a constant pointer called p, that is, p is the corresponding storage space is read-only, we can not change the content by P

p = &b; The correct wording

*p = 20; Wrong notation, because the const modifier is *p, so *p is read-only, and we can't write to read-only storage (p points to B is a variable, not a constant, but that's not possible)

b = 20; Correct notation, because B is a variable, no const modifier b

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int * Const Q = &a; Function: const is on the right side of *, declaring q is a pointer constant

Q = &b; Wrong notation, q is a read-only pointer variable and we cannot reassign it

*q = 20; Correct notation, because there is no const modification before *q, so Q point to the storage space is still readable, so *q is a variable, not a constant, we can modify the corresponding storage space by q

b = 20; The correct wording

We have to look at the location of the const is different, so play a different role, we have to think more. Summary: 1. Const on the left side, you can not change the *p, that is, can not be assigned: *p = 20; 2. Const on the right side of the *, you cannot change the p, that is, you cannot assign this value: P = &b;

3. Some of the cases of const:

1)

int main () {

const int a = 5;

int * p = (int *) &a;

*p = 10;

printf ("%d\n", a);

printf ("%d\n", *p);

}

We found out that a is a value of 5, and the value of *p is 10. Why is there such a situation? is storage space storing a different two values? Of course not, because when the compiler is preprocessing, only the const-modified variable value is read once, so print a is 5, actually we change the value of a by the pointer variable p, so the value of a is actually 10. Summary: 1. A const-Modified local variable is stored in the stack, and we can modify its value by pointer. 2. The compiler only reads the value of a const-modified variable once, at the time of preprocessing. 3. If a is a const-modified global variable, then this is not possible, because its value is stored in the global storage space, its value is only a readable property, cannot be modified.

In other words, it is not possible to write the following:

const int a = 5;

int main () {
int * p = (int *) &a;

*p = 10; It is wrong to write this way.

printf ("%d\n", a);

printf ("%d\n", *p);

}

2)

int a = 5;

int B = 10;

const INT * Const P = &a; Both the pointer itself and the storage space pointed to by the pointer are read-only.

p = &b; Wrong notation, because P is preceded by a const modifier, so p is a pointer constant, which means that the value of P cannot be re-assigned.

*p = 20; Wrong notation because *p has a const modifier in front of it, so *p is a constant, that is, p is a constant pointer, and the storage space it points to is read-only.

OK, the basic use of Const said here, we can go back to compare, serious thinking, I believe there will be no small harvest ...

The basic function and usage of the Const keyword in c language