Namespaces
Variants
Views
Actions

zero initialization

From cppreference.com
< cpp‎ | language
Revision as of 07:54, 18 March 2013 by Bazzy (Talk | contribs)

 
 
C++ language
General topics
Flow control
Conditional execution statements
Iteration statements
Jump statements
Functions
function declaration
lambda function declaration
function template
inline specifier
exception specifications (deprecated)
noexcept specifier (C++11)
Exceptions
Namespaces
Types
decltype specifier (C++11)
Specifiers
cv specifiers
storage duration specifiers
constexpr specifier (C++11)
auto specifier (C++11)
alignas specifier (C++11)
Initialization
Literals
Expressions
alternative representations
Utilities
Types
typedef declaration
type alias declaration (C++11)
attributes (C++11)
Casts
implicit conversions
const_cast conversion
static_cast conversion
dynamic_cast conversion
reinterpret_cast conversion
C-style and functional cast
Memory allocation
Classes
Class-specific function properties
Special member functions
Templates
class template
function template
template specialization
parameter packs (C++11)
Miscellaneous
Inline assembly
 

Sets the initial value of an object to zero

Contents

Syntax

static Template:sparam Template:sparam ; (1)
Template:sparam () ; (2)
char Template:sparam [ Template:sparam ] = ""; (3)

Explanation

Zero initialization is performed in the following situations:

1) For every named variable with static or thread-local storage duration, before any other initialization.
2) 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 that is too short, the remainder of the array is zero-initialized.

The effects of zero initialization are:

  • If T is a scalar type, the object's initial value is the integral constant zero implicitly converted to T.
  • If T is an non-union class type, all base classes and non-static data members are zero-initialized, and all padding is initialized to zero bits. The constructors, if any, are ignored.
  • If T is a union type, the first non-static named data member is zero-initialized and all padding is initialized to zero bits.
  • If T is array type, each element is zero-initialized
  • If T is reference type, nothing is done.

Notes

The static and thread-local variables are first zero-initialized and then initialized again as specified in the program, e.g. a function-local static is first zero-initialized at program startup, and then its constructor is called when the function is first entered. If the declaration of a non-class static has no initializer, then default initialization does nothing, leaving the result of the earlier zero-initialization unmodified.

A zero-initialized pointer is the null pointer value of its type, even if the value of the null pointer is not integral zero.

Example

#include <string>
 
double f[3]; // zero-initialized to three 0.0's
int* p;   // zero-initialized to null pointer value
std::string s; // zero-initialized to indeterminate value
               // then default-initialized to ""
int main(int argc, char* argv[])
{
    static int n = argc; // zero-initialized to 0
                         // then copy-initialized to argc
    delete p; // safe to delete a null pointer
}


See also