Failing compilation

by evaluating

a concept

Braden Ganetsky

CppNorth 2024 Lightning Talk

How I wish I could

constrain my

function template

Braden Ganetsky

CppNorth 2024 Lightning Talk

## This talk is about * Function templates * Putting conditions on function templates

Conditions on a function template

static_assert

Constraints

## Benefits of `static_assert` * User-specified diagnostic message

## Benefits of constraints * Callability evaluated as a predicate * Removes certain functions from overload set `foo`

Benefits of constraints

## If we used `static_assert` * `Fooable<double>` instantiates `foo<double>` * `foo<double>` hits the `static_assert` * Compilation failure from evaluating a concept (#1)

## What do I want? * User-specified diagnostic message * Also removing from the overload set * Is this possible?

What do I want?

## Idea #1 * `static_assert` inside the concept

Idea #1 - example

## Idea #1 - problems * The `static_assert` is never triggered (in MSVC at least)

## Idea #2 * Factor out the lambda

Idea #2 - example

## Idea #2 - problems * Same problems as a `static_assert` used regularly * Fundamental conflict: signature versus body

## Idea #3 * Credit to Patrick Roberts * Mix concepts with `enable_if` SFINAE

Idea #3 - example

## Idea #3 - problems * Lack of user-specified message * However, compilation failure from evaluating a concept (#2)

## Idea #4 * Custom `enable_if` implementation

Idea #4 - example

Idea #4 - example

## Idea #4 - problems * Almost completely satisfies what I want, however:

Idea #4 - problems

## Idea #4 - problems * It removes `foo<double>` from the overload set * I *still* don't get the error message I want

## Conclusion * I like writing constraints to check callability * I also like user-specified error messages * I want to mix them together * I haven't yet figured out how * ... * If you have an idea, please talk to me later!

Thank you!

Braden Ganetsky

braden@ganets.ky

GitHub @k3DW