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C++ Brain Teasers: Exercise Your Mind

Discussion: Hack the Planet!

Understand the consequences of using uninitialized local variables in C++ through examining stack behavior and undefined results. Learn how function call stacks reuse memory, why this leads to unexpected values, and the importance of initializing variables. Discover practical tools like compiler warnings, linting, and sanitizers to detect and avoid such errors, improving code reliability and safety.

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Run the code

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

C++ 17
#include <iostream>
int getUserId() { return 1337; }
void restrictedTask1()
{
    int id = getUserId();
    if (id == 1337) { std::cout << "did task 1\n"; }
}
void restrictedTask2()
{
    int id;
    if (id == 1337) { std::cout << "did task 2\n"; }
}
int main() 
{
    restrictedTask1();
    restrictedTask2();
}

Understanding the output

The id variable in the restrictedTask2()function has not been initialized and has an indeterminate value. To use its value is an undefined behavior. Anything can happen when a program runs into undefined behavior; the C++ standard makes no guarantees. Even the part of the program that happened before we took the value of id is undefined!

However, if we run this program, it will probably print both did task 1 and did task 2, at least if we compile without optimizations. So the value 1337 magically teleported from restrictedTask1() to restrictedTask2()! How could this ...