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Consider following code example:

#include <iostream> static int bar = bar; int main() { int foo = foo; std::cout << "foo = " << foo << std::endl; std::cout << "bar = " << bar << std::endl; }

I get following compiler warning:

main.cpp: In function 'int main()': main.cpp:7:15: warning: 'foo' is used uninitialized in this function [-Wuninitialized] int foo = foo; ^

Output:

foo = 0 bar = 0

I expected this warning as foo is used unitialised. Instead of 0, 'foo' can be anything. Self-assignment is undefined.

But why is the the self-assignment of 'bar' not warned? Is this assignment of 'bar' defined or undefined behaviour and why?

I know, static variables of elementar data types are initialised with '0' but in this case, the variable 'bar' is used during its initialisation. I'm wondering, if this is defined behaviour and the '0' is the expected output. (Which would explain, that no compiler warning occurs).

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  • Static initialization has more than one phase. Commented Apr 10, 2014 at 14:52
  • Probably because static variables in the global scope are always zero-initialized, so the compiler knows bar is effectively 0 before the assignment. Commented Apr 10, 2014 at 14:55
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    @FrédéricHamidi: Note that they are not zero-initialized, but default-constructed. That makes a difference for structs/classes! Commented Apr 10, 2014 at 14:58
  • @anderas, true, I should have said static primitives :) Commented Apr 10, 2014 at 14:59
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    @anderas AFAIK, static-storage-duration things are zero-initialised before any other initialisation takes place, including potential later execution of a default ctor. Commented Apr 10, 2014 at 16:05

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I believe that part of the standard is relevant to your question (§3.6.2/2):

Variables with static storage duration (3.7.1) or thread storage duration (3.7.2) shall be zero-initialized (8.5) before any other initialization takes place.[...]

So in this case, even before the compiler looks at your definition of bar, it has already zero-initialized it.

As it is specified a bit further in the standard, there shall be two phases for static variables initialization (emphasis mine).

Together, zero-initialization and constant initialization are called static initialization; all other initialization is dynamic initialization. Static initialization shall be performed before any dynamic initialization takes place.

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