how to construct a constexpr static_string from constexpr std::string_view

Question

I have a static_string template

template <std::size_t N>
struct static_string {
    std::array<char, N + 1> elements_; 

    constexpr static_string() noexcept : elements_{} {}

    constexpr static_string(const char (&data)[N + 1]) noexcept {
        for (size_t i = 0; i < N + 1; i++) {
            elements_[i] = data[i];
        }
    }
};

template <std::size_t N>
static_string(const char (&)[N]) -> static_string<N - 1>;

It works fine when use constexpr static_string str1("this is a str");

And I want to construct from a constexpr std::string_view, so I add:

    constexpr static_string(std::string_view sv) noexcept {
        for (size_t i = 0; i < N; ++i) {
            elements_[i] = sv[i];
        }
    }

then if I use

   constexpr std::string_view sv = "this is a str";
   constexpr static_string<sv.size()> str2(sv);

the compiler outputs

<source>: In function 'int main()':
<source>:33:47: error: 'static_string<13>{std::array<char, 14>{std::__array_traits<char, 14>::_Type{'t', 'h', 'i', 's', ' ', 'i', 's', ' ', 'a', ' ', 's', 't', 'r'}}}' is not a constant expression
   33 |     constexpr static_string<sv.size()> str2(sv);
      |                                               ^
<source>:33:47: error: 'static_string<13>(sv)' is not a constant expression because it refers to an incompletely initialized variable
Compiler returned: 1

So why this expression did not evaluate to a constant, is it possible to construct from constexpr std::string_view? thanks, see https://www.godbolt.org/z/6TToEPnch

I fixed a minor error and it solved the issue: godbolt.org/z/f1babx1r3. I don't make it an answer, because I don't get why adding the missing null termination also fixes the constexpr issue. — Oersted
– Oersted, Commented Jan 6 at 8:17

Oersted · Accepted Answer · 2025-01-06 10:25:30Z

9

The error message issued by the compiler is misleading.

In fact, the issue is not related to the input string_view but to the fact that elements_ is not fully initialized, which is not allowed in a constexpr constructor
<ref needed
perhaps https://timsong-cpp.github.io/cppwp/n4861/expr.const#11.4

A constant expression is... if the value is an object of class or array type, each subobject satisfies these constraints for the value.

>.

To fix that, just add the missing null terminating character:

#include <array>
#include <iostream>
#include <string_view>

template <std::size_t N>
struct static_string {
    std::array<char, N + 1> elements_;

    constexpr static_string() noexcept : elements_{} {}

    constexpr static_string(const char (&data)[N + 1]) noexcept {
        for (size_t i = 0; i < N + 1; i++) {
            elements_[i] = data[i];
        }
    }

    constexpr static_string(const std::string_view& sv) noexcept {
        for (size_t i = 0; i < N; ++i) {
            elements_[i] = sv[i];
        }
        elements_[N] = '\0';  // completing the initialization of the object
    }
};

template <std::size_t N>
static_string(const char (&)[N]) -> static_string<N - 1>;

int main() {
    constexpr std::string_view sv = "this is a str";
    constexpr static_string str1("this is a str");
    constexpr static_string<sv.size()> str2(sv);
}

LIVE

(btw, you can as well pass the string_view by const reference)

edited Jan 6 at 10:25

answered Jan 6 at 8:22

Oersted

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7 Comments

miyan Jan 6 at 9:19

really thanks! the fact that elements_ is not fully initialized, now I can use string_view and span, godbolt.org/z/5b9snxE3G

Caleth Jan 6 at 9:58

there's little need to pass a string view by const ref, it's already lightweight

Oersted Jan 6 at 10:24

@Caleth yes but isn't it better for readability and consistency of API?

Caleth Jan 6 at 10:50

No. Would you pass an int or a pointer by const ref?

Caleth Jan 6 at 10:59

Pointers aren't fundamental types, they are compound types. int and pointers are object types. I call std::string_view lightweight because it's a non-owning sequence. "A typical implementation holds only two members: a pointer to constant CharT and a size." cppreference

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