Zero Initialization in C++

Setting the initial value of an object to zero is called the zero initialization.

Syntax:

static T object;

Tt = {} ;

T {} ;

char array [n] = " ";

Zero initialization is performed in the following situations:-

  1. Zero is initialized for every named variable with static or thread-local storage duration that is not subject to constant initialization (since C++14), before any other initialization.
  2. Zero is initialied as part of value-initialization sequence for non-class types and for members of value-initialized class types that have no constructors.
  3. When a character array is initialized with a string literal which is very short, the remainder of the array is zero-initialized.

The effects of zero-initialization are:

  • If T is a scalar type, the object is initialized to the value obtained by converting the integer literal 0 to T.
  • If T is a non-union class type, ach non-static data member and each base-class subobject is zero-initialized and padding is initialized to zero bits.
  • If T is a union type, the object’s first non-static named data member is zero-initialized and padding is initialized to zero bits.
  • If T is an array type, each array element is zero-initialized.
  • If T is a reference type, no initilisation is performed.

Key Points:

  • The static and thread-local variables are first zero-initialized and then initialized again as specified in the program, e.g. in the starting of a program, function-local static is first zero-initialized, and then its constructor is called when the function is first entered. If there is no initialiser for the declaration of a non-class static, then default initialization does nothing, leaving the result of the earlier zero-initialization unmodified.
  • A pointer which is zero-initialized is called a null pointer, even if the value of the null pointer is not integral zero.

Below program illustrate zero initilisation in C++:

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// C++ code to demonstrate zero initialisation
  
#include <iostream>
#include <string>
  
struct foo {
    int x, y, z;
};
  
double f[3]; // zero-initialized to three 0.0's
  
int* p; // zero-initialized to null pointer value
  
// zero-initialized to indeterminate value
// then default-initialized to ""
std::string s; 
  
int main(int argc, char* argv[])
{
    foo x = foo();
      
    std::cout << x.x << x.y << x.z << '\n';
      
    return 0;
}

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Output:

000

Reference: https://en.cppreference.com/w/cpp/language/zero_initialization



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