initializing static variable with a function call gives compilation error?

Question

#include <stdio.h>
int foo(){
    return 1;
}
int main(void) {
    static int q = foo(); 
    return 0;
}

Here is a link for the same. This is a C code and not C++. It compiles and run fine in C++ but not C.

This code was getting compilation error. Can someone please explain why is it getting error? Can static members only be initialized by constant values ? In C++ we need to DEFINE static members after declaring them , why is it not required in C ? I couldn't find any thread with similar query or a good answer.

@iammilind Try with C. I am talking about C here. I will edit that in main body. — h4ck3d, Oct 04 '12 at 04:54
Not an exact duplicate, but close: http://stackoverflow.com/questions/3025050/error-initializer-element-is-not-constant-when-trying-to-initialize-variable-w — Matt, Oct 04 '12 at 05:01
@h4ck3d As below mentioned, Global and static variables can only be initialized with constant expressions known at compile time. so you can do, `int (*q) (void);q=foo;`. this should work in C. — overexchange, Oct 30 '14 at 04:44

Alexey Frunze · Accepted Answer · 2012-10-04T05:21:56.203

7

Global and static variables can only be initialized with constant expressions known at compile time. Calling your foo() function does not constitute using a constant expression. Further, the order in which global and static variables are initialized is not specified. Generally, calling foo() would mean that there must be a certain order, because the function can reasonably expect some other variables to be already initialized.

IOW, in C, neither of your code is executed before main().

In C++ there are ways around it, but not in C.

edited Oct 04 '12 at 05:21

answered Oct 04 '12 at 04:58

Alexey Frunze

61,140
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what is IOW ? in C++ first class loading takes place? – h4ck3d Oct 04 '12 at 05:25
1

I did not understand the thing about C++ classes, but in C++ global/static objects are constructed before `main()`, which gives you a chance to call a function at initialization time. There are some rules about the order of initialization in C++, or this wouldn't work in C++ either. – Alexey Frunze Oct 04 '12 at 05:27
@AlexeyFrunze Yes, it's just like in Java too. Like, one can print to `stdout` even before `main` is entered. By putting `System.out.println` call inside static initialization block. – John Strood Sep 25 '16 at 11:45

score 5 · Answer 2 · answered Oct 04 '12 at 07:27

All the static variables are compile time and the function is giving the output at run time so you are initializing a compile time variable with a run time variable which is not possible so it is giving error.

Another example may be as follows

int main()
{
int p=9;
static int x=p;
}

the above code is also gives you compile time error,The cause is same as above.

score 1 · Answer 3 · answered Oct 04 '12 at 05:05

1

If you are doing this in C rather than C++ you can only assign static variables values that are available during compilation. So the use of foo() is not permitted due to its value not being determined until runtime.

answered Oct 04 '12 at 05:05

Patrick Wilkin

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initializing static variable with a function call gives compilation error?

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