How to static_assert each data member of a C++ lambda?

Question

How can one call static_assert for each data member of any given C++ lambda?

What I'm trying to do is roll my own memcpy-able std::function and have realized that any lambdas must have trivially copyable data members. I'd like to put some assertions in place for this.

Example:

template<typename T> struct has_trivial_copy {enum {value = false};};
template<> struct has_trivial_copy<int> {enum {value = true};};

int main() {
    int i = 0;
    std::string str;

    auto lambda1 = [i] {
        // static_assert(has_trivial_copy<each data member...>::value); // okay
    };

    auto lambda2 = [i, str] {
        // static_assert(has_trivial_copy<each data member...>::value); // fail
    };
}

There is nothing built-in into C++ for that. It is up to you to call static_assert for whatever you want it to be called. — Sam Varshavchik
– Sam Varshavchik, Commented Oct 25, 2023 at 18:56
"What I'm trying to do is roll my own memcpy-able std::function and have realized that any lambdas must have trivially copyable data members.": Why do you need that? You only need to check std::is_trivially_copyable on the lambda type itself. — user17732522
– user17732522, Commented Oct 25, 2023 at 19:00
You cannot implement std::is_trivially_copyable in user C++ by the way. That requires magic compiler support. And having a trivial copy operation is not the same as being trivially copyable, which is what is required for memcpy. — user17732522
– user17732522, Commented Oct 25, 2023 at 19:02
if std::is_trivially_copyable_v<decltype(lambda2)> is true then the lambda does not have members that are not trivially copyable — 463035818_is_not_an_ai
– 463035818_is_not_an_ai, Commented Oct 25, 2023 at 19:03

Nathanael Weiss · Accepted Answer · 2023-10-25 19:53:57Z

0

While it's not possible to call static_assert for every member of a lambda, it is possible to use std::is_trivially_copyable on any given lambda to assert that it is trivially copyable.

I've added the following static_assert to my custom std::function and it works nicely to show me all the places where I'm using non-trivially copyable data members in lambdas:

template <typename Functor, typename ReturnType, typename... Args>
struct FunctorHolder
{
    FunctorHolder (Functor func) : f (func)
    {
        static_assert(std::is_trivially_copyable<Functor>::value);
    }

    Functor f;
};

answered Oct 25, 2023 at 19:53

Nathanael Weiss

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2 Comments

user17732522 Over a year ago

There is a difference between the type itself being trivially-copyable and all of its static members being trivially-copyable. In particular, it is up to the compiler to decide whether any given lambda is trivially-copyable, see eel.is/c++draft/expr.prim.lambda#closure-3.2. But since you intent to memcpy the lambda, not its members, checking the property of the lambda is the only correct thing to do anyway.

463035818_is_not_an_ai Over a year ago

@user17732522 "...and all of its static members being ..." -> non-static members

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