Const use in C
Const is a keyword of a C language that defines a variable that is not allowed to be changed. Using const can improve the security and reliability of the program to a certain extent.
Const's use is a subtle place in the C language, please see the following questions.
Problem: const variables and constants
Why is the following example to initialize an array using a const variable, will ANSI C report a bug?
Const int n = 5;
INT A [N];
Answer and analysis:
1) This problem discusses the difference between "constant" and "read-only variable". Constants are definitely read-only, for example 5, "abc", etc., must be read-only because constant is the read-only area where the compiler is placed in memory, and of course, it is not able to modify it. The "read-only variable" is a place to store it in memory, but this value is defined by the compiler that is not allowed to be modified.
C Language Keyword Const is used to qualify a variable that is not allowed to be changed. Qualifier. The variable N is modified as a read-only variable in the above code, but it is not a constant. And ANSI C specifies the length of the array to define the length of "constant" and "read-only variable" is also.
2) Note: In ANSI C, this way of writing is wrong because the size of the array should be a constant, and const INT N, N is just a variable (constant! = Non-variable variable, but in standard C , this definition Is a constant, this way is right), in fact, in accordance with the compilation process and memory allocation, this usage should be reasonable, just that the ANSI C pairs the array limit it.
3) So, what is the constant in the ANSI C language? The answer is an enum type and #define macro, both of which can be used to define constants.
Question: Const variables and constant content
The following code compiler will report an error. Which statement is wrong?
Typedef char * pstr;
Char string [4] = "ABC";
Const char * p1 = string;
Const PSTR P2 = String;
P1 ;
P2 ;
Answer and analysis:
The problem is in P2 .
1) Basic forms for use in Const: const char m;
Limited M unality.
2) Replace the M, Const char * PM in 1 form;
Limited * PM is not variable, of course, PM is variable, so P1 is pair in the problem.
3) Replace 1 CHAR, Const NewType M;
Limited M unality, Charptr in the problem is a new type, so P2 is not variable, and P2 is wrong.
Problem: const variables and string constants
What is the problem with the code below?
CHAR * P = "I'm Hungry!";
P [0] = 'I';
Answer and analysis:
The above code may cause illegal write operations of memory. The analysis is as follows, "I'm Hungry" is essentially a string, and constant is often placed in a read-only memory area and cannot be written. p Initially point to this read-only memory, and P [0] = 'i' attempts to write this place, the compiler will certainly not agree.
Problem: const variables and string constants 2
Can Char A [3] = "ABC" legal? What hidden dangers do it use? Answer and analysis:
This is legal in standard C, but its living environment is very narrow; it defines an array of size 3, which is initialized to "ABC", but it does not have a usual string terminator '\ 0', so this array is just It looks like a string in the C language, which is essentially, so all functions for processing the string, such as strcpy, printf, etc., can not be used on this false string.
Question 5: const and pointers
The type declaration is used to modify a constant, there are two ways to write, then, if you ask, what is the constant content with constant limited?
1) Const in front
Const int nvalue; // nvalue is const
Const char * pContent; // * PContent is const, PContent variable
Const (char *) pContent; // PContent is const, * pContent variable
Char * const pContent; // pContent is const, * pContent variable
Const char * const pContent; // pContent and * pContent are constant
2) Const is behind, the above statement is right
Int const nvalue; // nvalue is const
Char const * pcontent; // * PContent is const, PContent variable
(char *) const pContent; // pContent is const, * pContent variable
Char * const pContent; // pContent is const, * pContent variable
Char const * const pContent; // pContent and * pContent are constant
Answer and analysis:
CONST and pointers are used in C language a very common confusion in the actual development, especially when seeing others code, often because of this, not judge the author's intentions, hereby introducing an effective judgment in principle:
A, when there is "()", for example
Const (char *) PCONTENT;
At this time, (char *) is a whole, equivalent to a new type, therefore, then the defined pointer is const.
B, other cases
l On the * number, if the const is on the left side of *, the const is used to modify the variable pointed to by the pointer, that is, the constant refers to the pointer;
l If the const is on the right side of the *, const is the modified pointer itself, that is, the pointer itself is constant.
