Why is std::is_trivial deprecated in C++26?

Question

std::is_pod is deprecated since C++20 and the answer to this question tells us that we should use std::is_standard_layout + std::is_trivial instead.

However, with C++26, now also std::is_trivial is deprecated. Why is that and what should we use instead?

Answer 1

This change comes from P3247: Deprecate the notion of trivial types. The problem with std::is_trivial is that it mixes two different checks:

• std::is_trivially_copyable
• std::is_trivially_default_constructible

std::is_trivial is somewhat useless

These two checks are used in different situations, so having them bunched together into a single trait isn't very useful. For example, a container that explicitly manages lifetimes of objects stored in a std::byte[] (likestd::vector)

• can reorder/delay initialization for trivially default-constructible types,
• can copy its contents via std::memcpy for trivially copyable types, and
• can copy-assign its contents via std::memcpy for trivially copy-assignable types.

Knowing whether a type is trivial is somewhat useless because we're typically interested in more specific properties. Of course, there are some rare situations where we want to know both properties and std::is_trivial provides a bit of convenience, but it's not a lot of work to just ask for trivial copyability and trivial default construction separately.

std::is_trivial may be a pitfall

A developer may assume that if std::is_trivial is true, a type is "totally trivial" in every regard, including assignment. However, the proposal provides a counter-example:

struct S {
  const int i;
};

S is trivially copyable, yet it is not assignable. A user seeking to replace copy-assignment with memcpy should use is_trivially_copy_assignable for the check; checking just is_trivial or is_trivially_copyable as a precondition for assignment would underconstrain the user code.

Note that S is trivial, in case this wasn't clear.