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Question #14 Difficulty: 2

According to the C++23 standard, what is the output of this program? 

    #include <iostream>

class A {
public:
  A() { std::cout << "a"; }
  ~A() { std::cout << "A"; }
};

class B {
public:
  B() { std::cout << "b"; }
  ~B() { std::cout << "B"; }
};

class C {
public:
  C() { std::cout << "c"; }
  ~C() { std::cout << "C"; }
};

A a;

void foo() { static C c; }
int main() {
  B b;
  foo();
}

Correct!

§[basic.start.dynamic]¶5 in the standard:

It is implementation-defined whether the dynamic initialization of a non-block non-inline variable with static storage duration is sequenced before the first statement of main or is deferred. If it is deferred, it strongly happens before any non-initialization odr-use of any non-inline function or non-inline variable defined in the same translation unit as the variable to be initialized.

Since A() is not constexpr, the initialization of a is dynamic. There are two possibilities:
- a is initialized before main() is called.
- The initialization of a is deferred, but it is initialized before any non-inline function in the same translation unit as it is used. And since main() and a are defined in the same translation unit, it is still initialized before main().

When execution reaches B b, it is initialized as normal. Static local variables are initialized the first time control passes through their declaration, so c is initialized next. As main() is exited, its local variable b goes out of scope, and is destroyed. Finally, all static variables are destroyed in reverse order of their initialization, first c, then a.

You can explore this question further on C++ Insights or Compiler Explorer!

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