Can I Use std::string in Constant Expressions?

Utilizing std::string in Constant Expressions

Constexpr variables are a crucial component of modern C , offering compile-time resolution for complex expressions. However, due to their stringent requirements, using std::string in constexpr contexts has historically been problematic.

In C 11, attempts to define a constant std::string, as exemplified in the provided code snippet, result in compilation errors. The explanation lies in the non-trivial destructor of std::string, which prevents it from being considered a literal type.

C 20 and C 17 Solutions

Fortunately, C 20 introduced a solution to this dilemma. If the std::string is assured to be destroyed before constant evaluation concludes, it becomes valid to use within constexpr. While the example in the original query remains invalid, code such as determining the size of a std::string created from a non-constant string literal (as illustrated in the answer) compiles successfully.

In addition to C 20's enhancements, C 17 brought forth std::string_view. This string-like object provides an immutable reference to existing character sequences without owning or modifying them. Defining a constexpr std::string_view, as demonstrated in the answer, resolves the problem of using character strings in constant expressions.

Through these solutions, both C 20 and C 17 significantly enhance the expressiveness and power of constexpr in modern C programming.

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