Difference between initialization of static variab

Question

I was going through the code at http://geeksforgeeks.org/?p=10302

#include<stdio.h>
int initializer(void)
{
    return 50;
}

int main()
{
    static int i = initializer();
    printf(" value of i = %d", i);
    getchar();
    return 0;
}

This code will not compile in C because static variables need to be initialised before main() starts. That is fine. But this code will compile just fine in a C++ compiler.

My question is why it compiles in a C++ compiler when static has the same usage in both languages. Of course compilers will be different for these languages but I am not able to pin point the exact reason. If it is specified in the standard, I would love to know that.

I searched for this question on SO , found 3 similar links but in vain. Link1 Link2 Link3

Thanks for your help.

Answer 1

Static variables in C need to be initialised with a value known at compile time. This requirement has been removed in C++, and you can initialise them with expressions evaluated at run-time.

The two languages differ in this, and many, many other respects. You can quite easily write C code which will be acceptable to a C++ compiler, but the reverse is not true.

Answer 2

6.7.8/4 [C99]

All the expressions in an initializer for an object that has static storage duration shall be constant expressions or string literals.

In static int i = initializer(); the RHS is not a constant expression and so the code doesn't compile in C.

In C++ there is no such restriction and the code is well-formed in C++.

Answer 3

It compiles in C++ because C++ needs to support dynamic initialization anyway, or you couldn't have local static or non-local objects with non-trivial constructors.

So since C++ has this complexity anyway, supporting that initialization like you show isn't complicated to add anymore.

In C that would be a big matter because C doesn't have any other reason to support initialization done at program startup (apart from trivial zero initialization). In C, initial values of file-scope or local static objects can always statically be put into the executable image.