如何使用C中先前定义的常量定义静态const？

Question

This isn't quite answered by static const vs #define or #defining constants in C++ .

In when I did a lot of programming in C, ANSI C was brand new. We mostly used #define to define constants. I've been made aware that this is no longer a best practice ( https://codereview.stackexchange.com/questions/123848/verifying-e-mail-address-in-c/123856#123856 ).

Using #define I can use previous constants to define the current constant, an exampe is

#define EMAIL_CHAR_ARRAY_SIZE 40
#define GOOD_EMAIL_ADDRESS  1
#define BAD_EMAIL_ADDRESS 0
#define MIMIMUM_USER_NAME_LENGTH 1
#define AT_SIGN_LENGTH 1
#define DOT_LENGTH 1
#define MINIMUM_DOMAIN_LENGTH 1
#define MINIMUM_ROOT_DOMAIN_LENGTH 2
#define MINIUMUM_EMAIL_LENGTH MIMIMUM_USER_NAME_LENGTH + AT_SIGN_LENGTH + MINIMUM_DOMAIN_LENGTH + MINIMUM_ROOT_DOMAIN_LENGTH

I have a program (below) where I have attempted to use static const rather than #define, however the constant MINIUMUM_EMAIL_LENGTH doesn't compile when I try to use static const using the previous constants. Is there a way to use previously defined constants in a static const TYPE declaration?

const_testemail.c:12:5: error: initializer element is not constant
     static const int MINIUMUM_EMAIL_LENGTH = (MIMIMUM_USER_NAME_LENGTH + AT_SIGN_LENGTH + MINIMUM_DOMAIN_LENGTH + MINIMUM_ROOT_DOMAIN_LENGTH);


#include <stdio.h>
#include <string.h>

static const int EMAIL_CHAR_ARRAY_SIZE = 40;
static const int GOOD_EMAIL_ADDRESS  = 1;
static const int BAD_EMAIL_ADDRESS = 0;
static const int MIMIMUM_USER_NAME_LENGTH = 1;
static const int AT_SIGN_LENGTH = 1;
static const int DOT_LENGTH = 1;
static const int MINIMUM_DOMAIN_LENGTH = 1;
static const int MINIMUM_ROOT_DOMAIN_LENGTH = 2;
/*  Doesn't compile 
 *  static const int MINIUMUM_EMAIL_LENGTH = MIMIMUM_USER_NAME_LENGTH + AT_SIGN_LENGTH + MINIMUM_DOMAIN_LENGTH + MINIMUM_ROOT_DOMAIN_LENGTH;
 *  */

#define MINIUMUM_EMAIL_LENGTH (MIMIMUM_USER_NAME_LENGTH + AT_SIGN_LENGTH + MINIMUM_DOMAIN_LENGTH + MINIMUM_ROOT_DOMAIN_LENGTH)

int isEmailAddressProper(const char EmailAddress[EMAIL_CHAR_ARRAY_SIZE])
{
    int     EmailAddressIsGood = GOOD_EMAIL_ADDRESS;
    int     LengthOfEmailAddress;
    char    *AtSignLocation, *pos2;
    int     rootDomainLength;

    LengthOfEmailAddress = strlen(EmailAddress);
    if (LengthOfEmailAddress < MINIUMUM_EMAIL_LENGTH)
    {
        printf("The length of the email address is less than the minimum lenght %d\n", MINIUMUM_EMAIL_LENGTH);
        EmailAddressIsGood = BAD_EMAIL_ADDRESS;
    }

    AtSignLocation = strchr(EmailAddress, '@'); /* get the first instance of @ */
    if (!AtSignLocation)
    {
        printf("There is no @ in the email address\n");
        EmailAddressIsGood = BAD_EMAIL_ADDRESS;
        return EmailAddressIsGood;
    }

    if (AtSignLocation == EmailAddress) { /* Is @ the first character? */
        printf("There is no user name in the email address, @ is the first character\n");
        EmailAddressIsGood = BAD_EMAIL_ADDRESS;
    }

    pos2 = strrchr(EmailAddress, '@'); /* find any other @ */
    if ((pos2) && (AtSignLocation != pos2)) {
        printf("There is more than 1 @ in the email address\n");
        EmailAddressIsGood = BAD_EMAIL_ADDRESS;
    }

    pos2 = strrchr(EmailAddress, '.'); /* get the last instance of '.' */
    if (AtSignLocation > pos2) /* is . before @ ? */
    {            printf("There is no root domain in the email address\n");
        EmailAddressIsGood = BAD_EMAIL_ADDRESS;
    }

    pos2++;
    rootDomainLength = LengthOfEmailAddress - ((int)(pos2 - EmailAddress));
    /* if root domain less than length 2 */
    if (rootDomainLength < MINIMUM_ROOT_DOMAIN_LENGTH)
    {
        printf("The root domain length (%d) is less than the minimum length required (%d) in the email address\n", rootDomainLength, MINIMUM_ROOT_DOMAIN_LENGTH);
        EmailAddressIsGood = BAD_EMAIL_ADDRESS;
    }

    return EmailAddressIsGood;
}

void GetAndValidateEmailAddress(char EmailAddress[EMAIL_CHAR_ARRAY_SIZE])
{
    int EmailAddressIsGood = BAD_EMAIL_ADDRESS;
    char TempEmail[EMAIL_CHAR_ARRAY_SIZE];

    while (!EmailAddressIsGood)
    {
        scanf("%39s", TempEmail);
        EmailAddressIsGood = isEmailAddressProper(TempEmail);
        if (!EmailAddressIsGood) {
            printf("The email address is not in the proper format, please re-enter the email address\n");
        }
    }

   strcpy(EmailAddress, TempEmail);
}

main()
{
    char EmailAddress[EMAIL_CHAR_ARRAY_SIZE];

    printf("Please enter the email address\n");
    GetAndValidateEmailAddress(EmailAddress);
    printf("The email address you entered is %s\n", EmailAddress);
}

Answer 1

C does not have symbolic constants other than enum-constants (which are always of type int ). #define is a macro and part of the pre processor. It is a textual replacement before the actual C language compilation.

const qualified variables are semantically still variables. The qualifier is a guarantee by the programmer he will not change the value. The compiler may rely on this guarantee. Breaking the contract invokes undefined behaviour, but it is not necessarily enforced by the run-time environment.

Static initialisers and array indexes at the file-level require a constant expression , thus the error. Briefly, a constant expression must yield a constant value at compile-time, thus you cannot use variables.

Note the array parameter in your function has a similar problem. But here you can use a variable length array. However, you either can use an empty length ( [] ) or pass the length as a preceeding parameter explicitly; global variables don't work:

int isEmailAddressProper(size_t len, const char EmailAddress[len])

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C++ is a different language with similar syntax/grammar, and quite some different semantics even for the same grammar. You often cannot apply knowledge from on language to the other.