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Showing content from https://en.cppreference.com/w/cpp/algorithm/../language/attributes/indeterminate.html below:

C++ attribute: indeterminate (since C++26)

Indicates that the variable or function parameter has an indeterminate value if it is not initialized.

[[indeterminate]] can be applied to the definition of a block variable with automatic storage duration or to a declaration of a parameter of a function declaration. The attribute specifies that the bytes comprising the storage of an object with automatic storage duration is initially indeterminate rather than erroneous.

If a function parameter is declared with [[indeterminate]], it must be declared in the first declaration of its function. If a function parameter is declared with [[indeterminate]] in the first declaration of its function in one translation unit and the same function is declared without [[indeterminate]] on the same parameter in its first declaration in another translation unit, the program is ill-formed, no diagnostic required.

The [[indeterminate]] attribute restores the undefined behavior that was implicitly introduced until C++26. It may make compilers consider a code path reading an indeterminate value unreachable.

void f(int);
 
void g()
{
    int x [[indeterminate]]; // indeterminate value
    int y;                   // erroneous value
 
    f(x); // undefined behavior
    f(y); // erroneous behavior
}
 
struct T
{
    T() {}
    int x;
};
 
void h(T a [[indeterminate]], T b)
{
    f(a.x); // undefined behavior when called below
    f(b.x); // erroneous behavior when called below
}
 
h(T(), T());

• C++26 standard (ISO/IEC 14882:2026):

• 9.12.7 Indeterminate storage [dcl.attr.indet]

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