Difference between revisions of "cpp/language/zero initialization"
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* [[cpp/language/copy_initialization|copy initialization]] | * [[cpp/language/copy_initialization|copy initialization]] | ||
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Revision as of 20:01, 2 November 2012
Sets the initial value of an object to zero
Contents |
Syntax
static Template:sparam Template:sparam ;
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(1) | ||||||||
Template:sparam () ;
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(2) | ||||||||
char Template:sparam [ Template:sparam ] = "";
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(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
Tis a scalar type, the object's initial value is the integral constant zero implicitly converted toT.
- If
Tis 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
Tis a union type, the first non-static named data member is zero-initialized and all padding is initialized to zero bits.
- If
Tis array type, each element is zero-initialized
- If
Tis 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
Run this code
#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 }
