Why Can't I Use `constexpr` with `std::vector` and `std::string` in C 20?

Understanding the Incompatibility of constexpr and Dynamic Memory Allocation with std::vector and std::string in C 20

Despite documentation indicating support for constexpr std::string and std::vector in Microsoft Visual Studio 2019 version 16.11.4, users may encounter errors when attempting to create these objects with constexpr. The reason for this lies in the limited support for constexpr allocation in C 20.

In C 20, transient allocation is permitted during constexpr evaluation. This means that memory allocated during constant evaluation must be completely deallocated by its conclusion. However, vectors and strings allocate memory dynamically, which can persist beyond the scope of constant evaluation, violating this rule.

The error message "the expression must have a constant value" indicates that the compiler cannot guarantee that the allocated memory will be deallocated by the end of constexpr evaluation. Consequently, the attempted creation of constexpr std::vector and std::string objects fails.

In contrast, std::array can be used with constexpr because it does not require dynamic memory allocation and its lifetime is guaranteed to be limited to the constant evaluation period.

To address this issue, ensure that you only attempt to create constexpr objects when their lifetimes are transient, meaning they are completely deallocated by the end of constant evaluation. This limitation restricts the use of dynamic memory allocation within constexpr contexts.

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