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

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

### Syntax

`[[indeterminate]]` |  |

### Explanation

`[[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.

### Notes

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.

### Example

Run this code

    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());

### References

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

  - 9.12.7 Indeterminate storage [dcl.attr.indet]

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