C++ assign static array with constexpr char array pointers at compile time?

Question

I currently have a function that generate the type name for an array type. It's currently using other code that already runs at compile time. For example, for a variable such as int data[4], the function returns the string int[4]:

template<typename Class, int Size>
constexpr auto getName(Class (&)[Size])
{
    // code that already runs at compile time:
    constexpr auto name = getName<Class>();
    constexpr auto length = getNumericString<Size>();
    constexpr auto size = getStrLen(name) + getStrLen(length) + 3;

    // code I would like to run at compile time:
    static char buffer[size] = {0};
    if (buffer[0] == 0) {
        auto i = 0, j = 0;
        while (name[j] != 0) {
            buffer[i++] = name[j++];
        }
        buffer[i++] = '[';
        j = 0;
        while (length[j] != 0) {
            buffer[i++] = length[j++];
        }
        buffer[i++] = ']';
    }
    return buffer;
}

Is it possible to write the bottom part of that function to run at compile time somehow? It's just putting two const char* together with the [ and ] characters to denote the array size. And if its possible, how?

Thanks!

Technically possible? Yes. Templates are Turing-complete. Read: Template Metaprogramming. But should you? — Ivan Rubinson
– Ivan Rubinson, Commented Aug 9, 2018 at 18:38
Possible duplicate: stackoverflow.com/questions/38955940/… — Ivan Rubinson
– Ivan Rubinson, Commented Aug 9, 2018 at 18:43
As long as it's limited to Code Golf and not used in production, I can live with that. There can be many issues from abusing ones programming language, especially when working in a team. — Ivan Rubinson
– Ivan Rubinson, Commented Aug 9, 2018 at 19:03

Julius · Accepted Answer · 2018-08-11 06:11:49Z

Yes, it is possible. The most tricky part is that you ask for a solution which returns a const char* (rather than something which owns the data).

As you notice in your example, the const char* needs a buffer to point to. This is tricky because constexpr functions are currently (C++17) not allowed to have a static constexpr char buffer[size]{/* whatever*/};. Instead, you can use a static data member of a templated helper class.

The following is a complete demonstration which I have tested with both clang 6.0.1 and GCC 8.1.1 using -std=c++14.

#include <cassert>
#include <cstddef>

#include <iostream>

template<class T>
constexpr const char* getName();

template<>
constexpr const char* getName<int>() {
  return "int";
}

template<std::size_t N>
constexpr const char* getNumericString();

template<>
constexpr const char* getNumericString<16>() {
  return "16";
}

constexpr std::size_t getStrLen(const char* str) {
  std::size_t ret = 0;
  while(str[ret] != '\0') ret++;
  return ret;
}

static_assert(getStrLen("") == 0, "");
static_assert(getStrLen("ab") == 2, "");
static_assert(getStrLen("4\0\0aaa") == 1, "");

struct Wrapper {
  const char* str;

  constexpr auto begin() const { return str; }
  constexpr auto end() const {
    auto it = str;
    while(*it != '\0') ++it;
    return it;
  }
};

template<class T, std::size_t size>
class Array {
 private:
  T data_[size]{};
 public:
  constexpr T& operator[](std::size_t i) { return data_[i]; }
  constexpr const T& operator[](std::size_t i) const { return data_[i]; }
  constexpr const T* data() const { return data_; }
};

template<std::size_t buffer_size, class... Args>
constexpr Array<char, buffer_size> cat(Args... args) {
  Array<char, buffer_size> ret{};

  std::size_t i = 0;
  for(auto arg : {Wrapper{args}...}) {
    for(char c : arg) ret[i++] = c;
  }

  return ret;
}

template<class T, std::size_t N>
struct StaticDataForConstexprFunction {
  static constexpr const char* name = getName<T>();
  static constexpr const char* length = getNumericString<N>();
  static constexpr std::size_t size = getStrLen(name) + getStrLen(length) + 10;

  using Buffer = Array<char, size>;
  static constexpr Buffer buffer = cat<size>(name, "[", length, "]\0");
};

template<class T, std::size_t N>
constexpr typename StaticDataForConstexprFunction<T, N>::Buffer StaticDataForConstexprFunction<T, N>::buffer;

template<class T, std::size_t N>
constexpr const char* getName(T (&)[N]) {
  return StaticDataForConstexprFunction<T, N>::buffer.data();
}

int main() {
  int foobar[16];
  constexpr auto res = getName(foobar);
  std::cout << res << std::endl;
}

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