XCode: Initializer element is not a compile-time constant

Question

I am writing a program in C using XCode. I don't use C much, usually I use C++. I've never used XCode before.

Compile error is quite simple, the following lines of code are not being treated as compile time constants by the compiler.

const double PI = 4.0 * atan(1.0);
const double TAU = 8.0 * atan(1.0);

I'm sure this is allowed in C++ 11, although I can't be certain, since I last used it some months ago.

My guess is that the XCode compiler / the C standard does not allow constants to be computed in this way.

Is there an alternative I can use? I don't much fancy the "define" alternative...

#define PI 4.0 * atan(1.0);

As this will (may?) cause unnecessary run-time overhead.

Answer 1

the following lines of code are not being treated as compile time constants by the compiler.

The compiler is right, because they are not compile-time constants: they both make calls to the runtime portion of the Standard C library.

I don't much fancy the "define" alternative...

That's right, #define is not an alternative, because it would force a constant to be re-evaluated in each expression that you have it.

Is there an alternative I can use?

Sure - you can use M_PI for a definition of the π constant ^* , and 2*M_PI for the TAU :

const double TAU = 2 * M_PI;

I'm sure this is allowed in C++ 11

This is correct: unlike C, C++ does not requite initializers to be compile-time constants.

How is it that calling atan() isn't allowed, but the mathematical operation 2 * M_PI is allowed?

This is because the standard requires compilers to perform all numeric operations on constant expressions during the compile time. However, a single runtime call, such as atan(...) , will "poisons" the whole thing, so the compiler will evaluate as much as it can, but the expression will remain a runtime expression, not a compile-time constant.

^* It is not standard, but many libraries define it anyway.

Answer 2

This is not allowed in C. Unlike C++, C requires global variables to be initialized by compile-time constants. atan(1.0) is not a compile-time constant because it requires calling the function atan() at runtime.

The simple solution is don't call atan() , just us the actual numeric values of pi and tau as your initializers:

const double PI = 3.141592653589793;
const double TAU = 2*PI;

Some math libraries also provide the constant M_PI for you already, so you can just that:

const double PI = M_PI;

but that's not standard C (C89 or C99), so don't rely on all implementations having that constant.