将 unsigned int 的最大值转换为 int 并期望结果为 -1 是否可移植？

Question

I've found a C++ code where we cast an unsigned int 32, initialized to its max value, into a signed int and expect it to be -1 . It works well on the tested compiler, but is it really portable?

int GetBusinessDataID()
{
    u32_t id = ~0;

    // Some code that may return a valid ID.

    return id; // Here we expect to return -1.
}

Answer 1

This is guaranteed for integral conversions since C++20; before C++20 this is implementation-defined.

If the destination type is signed, the value does not change if the source integer can be represented in the destination type. Otherwise the result is

• implementation-defined (until C++20)

• the unique value of the destination type equal to the source value modulo 2 ⁿ where n is the number of bits used to represent the destination type. (since C++20).