Simplest way to combine two strings at compile time with C++11

Question

I'm trying to concatenate two compile time strings:

constexpr const char* getString1() { return "abababa"; };
constexpr const char* getString2() { return "lalalal"; };
constexpr const char* getString3() { return getString1() + getString2(); }; //wont compile

Is there a modern, simple way to do this in C++?

Also see: http://stackoverflow.com/questions/28708497/constexpr-to-concatenate-two-or-more-char-strings — NathanOliver, Oct 14 '16 at 13:08
`#define STRING1 "abababa"` and `#define STRING2 "lalalalal"`, then `return STRING1;`, `return STRING2;` and finally `return STRING1 STRING2;`. — Sam Varshavchik, Oct 14 '16 at 13:09
"Urg, Macros" - Nothing wrong with macros ... as long as you use them for what they were made for. — UKMonkey, Oct 14 '16 at 13:15
When you consider all the alternatives, macros seem like a lowly option. They are more for C than C++. — Arnav Borborah, Oct 14 '16 at 13:17
After you decay them to `const char *` there is no way for sure. But even without it it does not seem to be possible. — Slava, Oct 14 '16 at 13:42
Given the insistence on a solution involving appending results of `constexpr` functions, this question has no possible answer, other than "it cannot be done". — Peter, Oct 14 '16 at 13:55

score 2 · Answer 1 · 2022-05-16T18:24:17.460

This is a C++17 solution using std::apply.

NOTE: my usage of auto and requires pushes this into C++20.

This is actually possible by leveraging the fact that std::apply is constexpr meaning that you can access and tuple-type, including arrays, in terms of their underlying parameter pack:

#include <tuple>
#include <array>
#include <iostream>

//helper function:
//create array without the last element provided
//used to remove '\0' in concat
template <std::size_t N>
constexpr std::array<const char, N> trim(
    auto head, auto... chars
) requires (sizeof...(chars) == N) {
    if constexpr (sizeof...(chars) == 0) {
        return std::array<const char, 0>{}; //ignore last element
    } else {
        return std::apply([&head](auto... trim_chars){
            return std::array<const char, N>{head, trim_chars...}; //recurse
        }, trim<N - 1>(chars...));
    }
}

//concatonate 2 arrays by converting them into param backs using std::apply
//this works because std::apply is a constexpr function
template <std::size_t N, std::size_t M>
constexpr std::array<const char, N + M + 1> concat(
    const std::array<const char, N + 1>& n,
    const std::array<const char, M + 1>& m
) {
    std::array<const char, N> trim_n = std::apply([](auto... args){return trim<N>(args...);}, n);
    return std::apply([&](auto... n_chars){
        return std::apply([&](auto... m_chars){
            return std::array<const char, N + M + 1>{n_chars..., m_chars...};
        }, m);
    }, trim_n);
}

//example strings
constexpr std::array<const char, 4> x{"abc"};
constexpr std::array<const char, 4> y{"def"};
constexpr std::array<const char, 7> z = concat<3, 3>(x, y);

int main() {

    std::cout << &x[0] << " + " << &y[0] << " = " << &z[0];

}

This outputs:

abc + def = abcdef

πάντα ῥεῖ · Answer 2 · 2016-10-14T13:52:04.460

1

Is there a modern, simple way to do this in C++?

The final answer is:

There is no way to do this actually at compile time using constexpr functions.

 return getString1() + getString2();

would try to add two pointers, which makes no sense for the compiler.

Any other attempts would require to inspect the actual character literals for concatenation (e.g. something like strcat()) which cannot be done at compile time.

The available alternatives are to use macros as pointed out in comments.

_{Maybe someone is able to do some black voodoo template magic, but I doubt it's worth the efforts anyway.}

edited Oct 14 '16 at 13:52

answered Oct 14 '16 at 13:39

πάντα ῥεῖ

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I heard Jeff Atwood likes Black voodoo's. Anyway, even with that I doubt pointer concat is available. – Hatted Rooster Oct 14 '16 at 13:46
@GillBates I doubt so also. But I've seen amazing things like working with counting templates, deduced arrays, offsetof, etc. – πάντα ῥεῖ Oct 14 '16 at 13:46
I don't mind if the concatenation works with macros as long as they take input from the function returns. – user5024425 Oct 14 '16 at 13:49
That is not possible. – Peter Oct 14 '16 at 13:50
@user5024425 As mentioned, no way. Why is it so important it needs to be done at compile time? Is that a XY problem? – πάντα ῥεῖ Oct 14 '16 at 13:50
2

Here is some [template magic](http://stackoverflow.com/questions/28708497/constexpr-to-concatenate-two-or-more-char-strings) by Yakk to do just that :) – Rakete1111 Oct 14 '16 at 15:50
@Rakete1111 Hey, great. That's the kind of what I had in mind. Though, I have doubts it's worth it :) – πάντα ῥεῖ Oct 14 '16 at 15:52
@πάνταῥεῖ It is *definitely* not worth the effort :) – Rakete1111 Oct 14 '16 at 15:53

Simplest way to combine two strings at compile time with C++11

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