Uninitialized variables and UB

• Initialized = The object is given a known value at the point of definition.
• Assignment = The object is given a known value beyond the point of definition.
• Uninitialized = The object has not been given a known value yet.

• Uninitialized
  • the object has not been given a known value yet (through any means, including assignment).
  • So, an object that is not initialized but is then assigned a value is no longer uninitialized (because it has been given a known value).
• Uninitialized variable
  • a variable that has not been given a known value (through initialization or assignment)

int x;

• x is uninitialized
• In most cases when no initializer is provided, the variable is default-initialized, and performs no actual initialization
• it’s a performance optimizationinherited from C when computers were slow

Undefined behavior (UB)

Using values of uninitialized variables can lead to unexpected results

• result of executing code whose behavior is not well-defined by the C++ language

#include <iostream>
 
int main()
{
	//define int variable x
	int x; // this variable is uninitialized because we haven't given it a value
 
	// print value of x to the screen
	std::cout << x <<'\n'; // who knows what we'll get!
	return 0;
}

• Computer will assign some unused memory to x , it will then send the value residing in that memory location to std::cout
• This may give issues
• A possible workaround

#include <iostream>
 
void doNothing(int&) // Don't worry about what & is for now, we're just using it to trick the compiler into thinking variable x is used
{
}
 
int main()
{
    // define an integer variable named x
    int x; // this variable is uninitialized
 
    doNothing(x); // make the compiler think we're assigning a value to this variable
 
    // print the value of x to the screen (who knows what we'll get, because x is uninitialized)
    std::cout << x << '\n';
 
    return 0;
}

• Using the value from an uninitialized variable is our first example of undefined behavior (UB)