clang-tidy - bugprone-bitwise-pointer-cast — Extra Clang Tools 21.1.8 documentation

# Extra Clang Tools 21.1.8 documentation

## clang-tidy - bugprone-bitwise-pointer-cast

# bugprone-bitwise-pointer-cast

Warns about code that tries to cast between pointers by means of `std::bit_cast` or `memcpy`.

The motivation is that `std::bit_cast` is advertised as the safe alternative to type punning via `reinterpret_cast` in modern C++. However, one should not blindly replace `reinterpret_cast` with `std::bit_cast`, as follows:

    int x{};
    -float y = *reinterpret_cast<float*>(&x);
    +float y = *std::bit_cast<float*>(&x);

The drop-in replacement behaves exactly the same as `reinterpret_cast`, and Undefined Behavior is still invoked. `std::bit_cast` is copying the bytes of the input pointer, not the pointee, into an output pointer of a different type, which may violate the strict aliasing rules. However, simply looking at the code, it looks “safe”, because it uses `std::bit_cast` which is advertised as safe.

The solution to safe type punning is to apply `std::bit_cast` on value types, not on pointer types:

    int x{};
    float y = std::bit_cast<float>(x);

This way, the bytes of the input object are copied into the output object, which is much safer. Do note that Undefined Behavior can still occur, if there is no value of type `To` corresponding to the value representation produced. Compilers may be able to optimize this copy and generate identical assembly to the original `reinterpret_cast` version.

Code before C++20 may backport `std::bit_cast` by means of `memcpy`, or simply call `memcpy` directly, which is equally problematic. This is also detected by this check:

    int* x{};
    float* y{};
    std::memcpy(&y, &x, sizeof(x));

Alternatively, if a cast between pointers is truly wanted, `reinterpret_cast` should be used, to clearly convey the intent and enable warnings from compilers and linters, which should be addressed accordingly.