Avoid complex initialization of global objects

Reason

Complex initialization can lead to undefined order of execution.

Example

// file1.c

extern const X x;

const Y y = f(x);   // read x; write y

// file2.c

extern const Y y;

const X x = g(y);   // read y; write x

Since x and y are in different translation units the order of calls to f() and g() is undefined; one will access an uninitialized const. This shows that the order-of-initialization problem for global (namespace scope) objects is not limited to global variables.

Note

Order of initialization problems become particularly difficult to handle in concurrent code. It is usually best to avoid global (namespace scope) objects altogether.

Enforcement

• Flag initializers of globals that call non-constexpr functions
• Flag initializers of globals that access extern objects