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C++ Brain Teasers: Exercise Your Mind

Discussion: All Good Things Must Come to an End

Explore the lifetime and initialization behaviors of global, local, and local static variables in C++. Understand dynamic initialization, object destruction order, and best practices for managing global variables to avoid common pitfalls.

We'll cover the following...

Run the code

Now, it’s time to execute the code and observe the output.

C++ 17
#include <iostream>
#include <string>
struct Connection
{
    Connection(const std::string &name) : name_(name)
    {
        std::cout << "Created " << name_ << "\n";
    }
    ~Connection() 
    {
        std::cout << "Destroyed " << name_ << "\n";
    }
    std::string name_;
};
Connection global{"global"};
Connection &get()
{
    static Connection localStatic{"local static"};
    return localStatic;
}
int main()
{
    Connection local{"local"};
    Connection &tmp1 = get();
    Connection &tmp2 = get();
}

A quick recap

In the Hack the Planet! and Going Global puzzles, we learned about the lifetime and initialization differences between a global variable and a local variable:

C++ 17
int id; // global variable, one instance throughout the whole program
void f() 
{
    int id = 2; // local variable, created anew each time `f` is called
}

In this puzzle, we add two new interesting aspects to the object lifetime:

  • We no longer initialize the variables to a simple constant; there is a constructor with a side effect.

  • We introduce a local variable localStatic with static storage duration.

So, how does the behavior change when we introduce a constructor and a local static variable?

Global variable

Let’s first consider the global variable global. Like the global variable id we saw previously, it has static storage duration, which means only one instance is alive from its initialization until its destruction at the very end of the program.

We previously saw that the global int id; without an initializer was zero-initialized. We also mentioned that a global with an initializer ...