Why do I get an error about the initializer not being a constant?

问题

I am using the following code.

const int X_ORIGIN = 1233086;             
const int Y_ORIGIN = -4728071;              
const int Z_ORIGIN = 4085704;
const int xyzOrigin[NUM_DIMENSIONS] = {X_ORIGIN, Y_ORIGIN, Z_ORIGIN};

When I compile it, GCC gives me the following error.

Transformations.h:16:1: error: initializer element is not constant

What does that mean? How can I fix my code?

回答1:

You can't do this at global scope in C, only at local scope, i.e. within a function:

#define NUM_DIMENSIONS 3

const int X_ORIGIN = 1233086;             
const int Y_ORIGIN = -4728071;              
const int Z_ORIGIN = 4085704;

const int xyzOrigin[NUM_DIMENSIONS] = {X_ORIGIN, Y_ORIGIN, Z_ORIGIN}; // FAIL

void foo(void)
{
    const int xyzOrigin[NUM_DIMENSIONS] = {X_ORIGIN, Y_ORIGIN, Z_ORIGIN}; // OK
}

Alternatively you could compile the code as C++ rather than C.

回答2:

Often people are mislead by the naming of the keyword const, implying something of a constant value that can't be changed. In C at least, it means readonly. const qualified objects at file scope are not having the proper constness to serve as array initializers.

As an example for non-constant constness, it is perfectly ok to declare

 const volatile unsigned int milliseconds_since_boot;

being a value that gets updated from outside the compiler's control (think HW register) and that you are not allowed to assign to, i.e. it is readonly.

回答3:

I'm not a proper programmer ;) but I'd do this:

#define X_ORIGIN (1233086)
#define Y_ORIGIN (-4728071)
#define Z_ORIGIN (4085704)
const int xyzOrigin[NUM_DIMENSIONS] = {X_ORIGIN, Y_ORIGIN, Z_ORIGIN};

That way it's just a text-substitution. If the compiler still spits the dummy at least you're a step closer to knowing where the issue is.

回答4:

As an alternative, this would also work in this case:

enum { X_ORIGIN = 1233086,
       Y_ORIGIN = -4728071,
       Z_ORIGIN = 4085704 };

const int xyzOrigin[] = { X_ORIGIN, Y_ORIGIN, Z_ORIGIN };

int main()
{
    return 0;
}

回答5:

In C language objects with static storage duration has to be initialized with constant expressions or with aggregate initializers containing constant expressions. --Answer of AndreyT

After reading, You must have the knowledge that NUM_DIMENSIONS, If it has the const-qualification, isn't a constant! Then you can't initializate your array this way.

For use this code:

const int xyzOrigin[NUM_DIMENSIONS] = {X_ORIGIN, Y_ORIGIN, Z_ORIGIN};

You should use: #define NUM_DIMENSIONS 3 or you could just declare without any variable inside the square brackets const int xyzOrigin[] = {X_ORIGIN, Y_ORIGIN, Z_ORIGIN};

回答6:

As triclosan said:

main()
{
    const int X_ORIGIN = 1233086;
    const int Y_ORIGIN = -4728071;
    const int Z_ORIGIN = 4085704;
    const int xyzOrigin[] = {X_ORIGIN, Y_ORIGIN, Z_ORIGIN};
}

this works fine...

or, if you know the dimensions beforehand, this:

#define DIM 3

main()
{
    const int X_ORIGIN = 1233086;
    const int Y_ORIGIN = -4728071;
    const int Z_ORIGIN = 4085704;
    const int xyzOrigin[DIM] = {X_ORIGIN, Y_ORIGIN, Z_ORIGIN};
}

来源：https://stackoverflow.com/questions/11541088/why-do-i-get-an-error-about-the-initializer-not-being-a-constant

标签

const

initializer