Problem with consteval and multiplying magic numbers. c++

Question

Alright, so I'm trying to make a function that will hash a string.

consteval int hash_string(const char* str)
{
    constexpr int magic_number = 13371337;
    
    int num1 = 1337;
    int num2 = 7331;
    
    //do stuffz here//
    
    return num1 + num2 * magic_number; //error
    return num1 + num2;                //okay :D
}

I'm assuming that it's due to an integer overflow and that the compiler is unable to solve for it. But it's part of the algorithm, I don't care about that.

I'm expecting that consteval will solve the function because it's all compile time stuff.

Signed integer overflow is undefined behavior. Compile-time evaluation doesn't do undefined behavior. Does the outcome change if you switch all your integers to unsigned? — Igor Tandetnik
– Igor Tandetnik, Commented Nov 1, 2023 at 17:44
If you want to be a bit more in control, use std::uint64_t instead of int — Pepijn Kramer
– Pepijn Kramer, Commented Nov 1, 2023 at 17:50
Also for consteval consider using const std::string_view instead of const char* (so you can avoid pointer arithmetic) — Pepijn Kramer
– Pepijn Kramer, Commented Nov 1, 2023 at 17:52
Integer overflow is undefined behaviour. This means the program is legally allowed to format your hard drive, eat your lunch, and steal your girlfriend. If you want any of that to be "a part of the algorithm", go ahead and use an algorithm that can result in integer overflow. If not, use unsigned types. Unsigned arithmetic doesn't overflow, it wraps around. — n. m. could be an AI
– n. m. could be an AI, Commented Nov 1, 2023 at 17:58

Nicol Bolas · Accepted Answer · 2023-11-01 18:07:34Z

2

But it's part of the algorithm, I don't care about that.

But you should care about it, because C++ doesn't allow it. Signed integer overflow is undefined behavior. And during constant evaluation, if you provoke undefined behavior, your program is ill-formed.

Hence the compile error.

If your algorithm relies on something whose results are not defined, your algorithm needs to be changed.

answered Nov 1, 2023 at 18:07

Nicol Bolas

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