How to create a constexpr array with a sequence of string_views at compile time?

Question

I would like to create a constexpr std::array<std::string_view, ConstexprNumber>. It should for example contain constexpr std::strings_view's like this:

"text0", "text1", "text2", ..... "textn"

I came up with the following initial solution:

#include <iostream>
#include <array>
#include <utility>
#include <string>
#include <string_view>

// Number of strings that we want to generate
constexpr size_t NumberOfTextsToGenerate = 10u;

// constexpr function to build a string
constexpr std::string_view makeString(unsigned int i) {
    return std::string_view("text");
}

// Helper: constexpr function that will create an array of string_views and initialize it
template <unsigned int... ManyIntegers>
constexpr auto generateTextHelper(std::integer_sequence<unsigned int, ManyIntegers...>) {
    return std::array<std::string_view, sizeof...(ManyIntegers)>{ {makeString(ManyIntegers)...}};
}

// Helper: constexpr function that will return an array of string_views as shown above with a specified number of texts
constexpr auto generateTextArray() {
    return generateTextHelper(std::make_integer_sequence<unsigned int, NumberOfTextsToGenerate>());
}

// This is a definition of a std::array<std::string_view,UpperBound> initialized with some text
constexpr auto text = generateTextArray();

int main() {
    for (size_t i{}; i < NumberOfTextsToGenerate; ++i)
        std::cout << text[i] << '\n';
    return 0;
}

But, of course the "makeString" function does not do what I want. Or, better said, I do not know how to implement the correct solution.

How can we get such an array to work? Either based on this initial solution or something similar?

Answer 1

UPDATE: Another solution through STL.

The below solution is through boost's preprocessing macros.

---

Well, it took me three hours, but I still couldn't do it successfully through STL, so I gave up and eventually I went back to the macro.

If you don't want to import the whole boost library, you can just separate out these BOOST_PP macros into your project, although they are still very large, so it might take a bit of your time.

Here is the code (link to godbolt):

#include <iostream>
#include <string_view>
#include <experimental/array>

#include <boost/preprocessor/punctuation/comma_if.hpp>
#include <boost/preprocessor/repetition/repeat.hpp>


int main()
{
    constexpr auto strs_array = std::experimental::make_array<std::string_view>
    (
#define ADD_STRING(z, index, data)    BOOST_PP_COMMA_IF(index) data # index
        BOOST_PP_REPEAT(20, ADD_STRING, "test")
#undef ADD_STRING
    );

    for (const std::string_view &str : strs_array) {
        std::cout << str << '\n';
    }
}

The maximum number allowed to be generated is BOOST_PP_LIMIT_REPEAT (currently is 256), which means you can currently generate up to "test255", I guess that's enough.

Output:

Program returned: 0
Program stdout
test0
test1
test2
test3
test4
test5
test6
test7
test8
test9
test10
test11
test12
test13
test14
test15
test16
test17
test18
test19