constexpr vs static constexpr

Jason Turner mentioned that for data at function scope, static constexpr should be preferred solution over constexpr in most cases. ¹

The reason is that constexpr values are regular stack values ¹. To demonstrate that constexpr variables are stack variables, consider the following example:

const int* p = nullptr;
{
  constexpr std::array<int, 1000> value = get_value();
  p = &values[985];
}
// p is dangled
// If we enable asan in unoptimzed build, we will get a warning if we use p here

Instead, static constexpr will change the variable to static storage duration. ¹

Performance Trade off

There may be a performance trade-off to consider. For small data sizes, the constexpr version may offer better performance in release builds on GCC. However, for larger datasets, the static constexpr version tends to be significantly faster. The static version is also always faster on Clang or in debug build ². If performance is a concern, benchmarking is recommended.

Global Variables

For global variables, a (static) constexpr variable in a header file will cause duplication across compilation units . Instead, using inline constexpr is a better choice. ³

Footnotes

1. C++ Weekly - Ep 312 - Stop Using `constexpr` (And Use This Instead!) - YouTube ↩ ↩² ↩³

2. C++ Weekly - Ep 315 - constexpr vs static constexpr - YouTube ↩

3. C++ Weekly - Ep 339 - `static constexpr` vs `inline constexpr` - YouTube ↩