Sets the initial value of an object to zero.
static T object ; | (1) | |
T () ; T t | (2) | (since C++11) |
CharT array [ n ] = ""; | (3) |
Zero initialization is performed in the following situations:
The effects of zero initialization are:
T
is a scalar type, the object's initial value is the integral constant zero explicitly converted to T
. 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. T
is a union type, the first non-static named data member is zero-initialized and all padding is initialized to zero bits. T
is array type, each element is zero-initialized T
is reference type, nothing is done. As described in non-local initialization, static and thread-local variables that aren't constant-initialized (since C++14) are zero-initialized before any other initialization takes place. If the definition of a non-class non-local variable 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.
#include <string> #include <iostream> struct A { int a,b,c; }; 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[]) { A a = A(); std::cout << a.a << a.b << a.c << '\n'; static int n = argc; // zero-initialized to 0 // then copy-initialized to argc delete p; // safe to delete a null pointer }
Output:
000
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR | Applied to | Behavior as published | Correct behavior |
---|---|---|---|
CWG 2026 | C++14 | zero-init was specified to always occur first, even before constant-init | no zero-init if constant init applies |
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