How to use static_assert inside of a constexpr

Question

I have a function that can be called on an enum, save for one value. I want to check statically that the function is not called with that value. I have the following (simplified) (c++11) code:

enum class Channel: char {
    ALL = 0,
    ONE = 1,
    TWO = 2,
};

inline constexpr std::size_t zeroBasedChannelIndex(const Channel channel) {
    static_assert(channel != Channel::ALL, "Channel::All doesn't map to an index");
               // ^^^^^^^ error location
    return static_cast<std::size_t>(channel) - 1;
}

I'm getting the following error though: the value of parameter "channel" (declared at line 16 of "/opt/kvanwel/Workspaces/acdipole/amplib/src/powersoft/kseries/datamodel/../constants/Channel.h") cannot be used as a constant

It looks to me that channel IS constant... but computer says no. What am I doing wrong here?

static_assert can't depend on function arguments it doesn't matter if function is a constexpr. static_assert is for time when function is compiled (or when template is instantiated), not when it is executed (even if it is executed during compile time - when compiling other function/code). — Marek R
– Marek R, Commented Dec 12, 2022 at 9:39
It doesn't matter if channel is taken as a const. You can call the function with any Channel value. — Fareanor
– Fareanor, Commented Dec 12, 2022 at 9:41
Even when it's an inline constexpr? I thought inline meant that the code at the caller site was replaced with the code inside of the function... — Typhaon
– Typhaon, Commented Dec 12, 2022 at 9:43

catnip · Accepted Answer · 2022-12-12 14:42:37Z

3

The comments above have told you what's wrong. The only thing you can do to get round this is to pass channel as a template parameter:

template <Channel channel> constexpr std::size_t zeroBasedChannelIndex () {
    static_assert(channel != Channel::ALL, "Channel::All doesn't map to an index");
    return static_cast<std::size_t>(channel) - 1;
}

But now, that parameter must be known at compile time (i.e., in practise, a constant, most likely) when calling the function.

edited Dec 12, 2022 at 14:42

answered Dec 12, 2022 at 9:45

catnip

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