shared c constants in a header

问题

I want to share certain C string constants across multiple c files. The constants span multiple lines for readability:

const char *QUERY = "SELECT a,b,c "
                    "FROM table...";

Doing above gives redefinition error for QUERY. I don't want to use macro as backspace '\' will be required after every line. I could define these in separate c file and extern the variables in h file but I feel lazy to do that.

Is there any other way to achieve this in C?

回答1:

In some .c file, write what you've written. In the appropriate .h file, write

extern const char* QUERY; //just declaration

Include the .h file wherever you need the constant

No other good way :) HTH

回答2:

You could use static consts, to all intents and purposes your effect will be achieved.

myext.h:

#ifndef _MYEXT_H
#define _MYEXT_H
static const int myx = 245;
static const unsigned long int myy = 45678;
static const double myz = 3.14;
#endif

myfunc.h:

#ifndef MYFUNC_H
#define MYFUNC_H
void myfunc(void);
#endif

myfunc.c:

#include "myext.h"
#include "myfunc.h"
#include <stdio.h>

void myfunc(void)
{
    printf("%d\t%lu\t%f\n", myx, myy, myz);
}

myext.c:

#include "myext.h"
#include "myfunc.h"
#include <stdio.h>

int main()
{
    printf("%d\t%lu\t%f\n", myx, myy, myz);
    myfunc();
    return 0;
}

回答3:

You can simply #define them separate

#define QUERY1 "SELECT a,b,c "
#define QUERY2 "FROM table..."

and then join them in one definition

#define QUERY QUERY1 QUERY2

回答4:

There are several ways

place your variables in one file, declare them extern in the header and include that header where needed
consider using some external tool to append '\' at the end of your macro definition
overcome your laziness and declare your variables as extern in all your files

来源：https://stackoverflow.com/questions/5499504/shared-c-constants-in-a-header

标签

header

constants

string-literals