Question

Here’s my code:

main.cpp

#include "color.h"

int main ( int argc , char **argv ) 
{
    Highlight h;
    return 0;
}

color.h

#ifndef HIGH_H
#define HIGH_H
#include <iostream>

using namespace std;

struct colorPattern_t
{
    int c1 , c2;

    colorPattern_t ( int a , int b )
    {
        c1 = a; c2 = b;

        cout << "colorPattern_t() with " << c1 << " , " << c2 << endl;
    }

    void say()
    {
        cout << "say() from colorPattern_t" << endl;
    };
};

class Highlight
{
    public:
    Highlight ();
};

#endif

Now color.cpp

#include "color.h"

extern colorPattern_t colors[2] = 
{
    {
        1,
        2
    } ,
    {
        4,
        5
    }
};

Highlight::Highlight()
{
    for ( int i = 0 ; i < sizeof(colors) / sizeof(colorPattern_t) ; i ++ )
    {
        colors[i].say();
    }
};

When compiling with:

g++ main.cpp color.cpp -o main

I see:

color.cpp:3:31: warning: ‘colors’ initialized and declared ‘extern’
color.cpp:13:1: warning: extended initializer lists only available with -std=c++0x or -std=gnu++0x
color.cpp:13:1: warning: extended initializer lists only available with -std=c++0x or – std=gnu++0x

I there any suggestion for my colorPattern_t{} initialization method? I wanted them to be mounted in my code without using -std=c++0x to fix the symptom.

Answer 1

In your example, I don’t see anything outside of color.cpp accessing colors, so there is no reason to have the extern on its definition.

If other files are going to have access to this global data, then declare in colors.h that there exists a colors data structure:

extern colorPatterns_t colors[];

Then in your color.cpp drop the extern and initialize it as you’re doing. The extern tells the compiler that the variable is declared (and possibly initialized) somewhere else.

If you do access colors from another file outside of color.cpp, the sizeof colors won’t work, so you’ll have to signal its size in some other way. Either define the number of colors

 /* in colors.h */
 #define N_COLORS 2
 extern colorPatterns_t colors[];

 /* in color.cpp */
 colorPatterns_t colors[N_COLORS] = { ...

Or you can put in a marker in the last slot (e.g. -1 or some other obvious illegal value).

Of course, global variables are in general evil and you should just supply a set of routines/methods in color.c to access / manipulate the colors structure so that you’re free to change or update its implementation without affecting the rest of your code base.