How can C++ avoid complex initialization of global objects

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Reason

Complex initialization may result in undefined execution order.

Example

// file1.cextern const X x;const Y y = f(x); // read x; write y// file2.cextern const Y y;const X x = g(y); // read y; write x

Since x and y are in different translation (compilation) units, the order in which f() and g() are called is undefined; one of them accesses an uninitialized constant. This indicates that initialization order problems that occur with global (namespace-scoped) objects are not limited to global variables.

Translator's Note: This also affects the functions that use them.

Note:

In concurrent code, initialization order issues are particularly difficult to handle. In summary, the best option in general is to avoid global (namespace-scoped) objects. Enforcement

Flag initializers of globals that call non-constexpr functions

Marks global objects that call initialization of non-constant expression functions.

Constant expression functions may be evaluated at compile stage and therefore safe.

Flag initializers of globals that access extern objects

Marks global objects initialized with external variables.

External variables can be initialized in unpredictable ways.

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