Compiler Warning (level 4) C4463

overflow; assigning value to bit-field that can only hold values from low_value to high_value

The assigned value is outside the range of values that the bit-field can hold. Signed bit-field types use the high-order bit for the sign, so if n is the bit-field size, the range for signed bit-fields is -2^n-1 to 2^n-1-1, while unsigned bit-fields have a range from 0 to 2ⁿ-1.

Example

This example generates C4463 because it attempts to assign a value of 3 to a bit-field of type int with a size of 2, which has a range from -2 to 1.

To fix this issue, you can change the assigned value to something in the allowed range. If the bit-field is intended to hold unsigned values in the range from 0 to 3, you can change the declaration type to unsigned. If the field is intended to hold values in the range -4 to 3, then you can change the bit-field size to 3.

// C4463_overflow.cpp
// compile with: cl /W4 /EHsc C4463_overflow.cpp
struct S {
    int x : 2; // int type treats high-order bit as sign
};

int main() {
    S s;
    s.x = 3; // warning C4463: overflow; assigning 3
    // to bit-field that can only hold values from -2 to 1
    // To fix, change assigned value to fit in range,
    // increase size of bitfield, and/or change base type
    // to unsigned.
}