According to legend, in the world of C there is a superb equipment (tie) const, its appearance, so that all the assassins and hackers in the world are afraid, under its protection, all variables can be saved intact const is The abbreviation of constant means that the sea is dead, constant and constant, once accompanied and never changed. As long as a variable is preceded by const, it means that the variable's data can be accessed and cannot be modified. In fact, we can also call it readonly.
Although understanding const is relatively easy to understand, const can be used not only for the rhetorical basic types, but it can also be used frequently to make rhetoric some construction types and pointers and their associated entities, such as arrays, pointers, pointer arrays, structure arrays, Once the structure pointer array is combined with these complex types, it is still somewhat confusing.
E.g:
(1) const int a = 10;
(2) int const a = 10;
(3) const int a[10] = {1,2,3, 4, 5, 6, 7, 8, 9, 10};
(4) const int *p;
(5) int * const p;
(6) const struct devices dev[5];
(7) struct devices const * dev[5];
Seeing the above example, I believe many people will breathe cool. Talk about identification skills directly
Remove the type;
Who see const modification, who will have the iron shirt, whose value can not be modified, is readonly (1) remove the type int becomes "const a = 10", a has a iron shirt, a value is unchanged (2) remove the type int becomes "const a = 10", a has a iron shirt, the value of a is the same, the effect of these two is the same (3) remove the type int becomes "const a [10 ]â€, a[10] owns a T-shirt, the value in a array is unchanged (4) removes the type int becomes “const *pâ€, *p owns the iron blouse, the space in p refers to The value is unchanged (5) Remove the type int * becomes "const p", the pointer variable p has a T-shirt, the value of the pointer variable p does not change, that is to say p can no longer point to other addresses, but the p The value of the pointed-to space is variable (6) The type struct devices are removed and they are changed to "const dev[5]". dev[5] has a T-shirt and the value of the dev[5] array is unchanged (7) This is a device An array of pointers to the structure type. It has pointers to five types of devices. Each pointer points to a devices structure. Const modifies *dev[5] and removes the type struct devic. Es becomes "const *dev[5]", and the pointer array *dev[5] has a T-shirt, and the value in the space pointed to by each element in the pointer array dev is unchanged.
Function summary:
First, when the const keyword decorates a variable name, this variable becomes a read-only variable, and the read-only variable is not the or section of the global data area. The variable it modifies is originally in which area, after const modification it is still in that area. For example, a local variable const int num = 5; then this local variable still exists on the stack space. Its corresponding memory space can still be changed. But it cannot be modified by num. If you define a pointer to the address of num. You can change the value of num. Secondly, const can modify parameters (protected parameters are not modified). At this point, from the perspective of the implementer, const can prevent problems caused by the modification of the actual parameters. From the caller's point of view, it can only be accessed for actual arguments and cannot be modified. Therefore, the rational use of the const keyword allows the compiler to naturally protect those parameters that do not wish to be changed, preventing it from being inadvertently modified. In short, this can reduce the appearance of bugs.
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