c++ numerical parser using template metaprogramming - c++

Question

i have been working for around 4 hours tryng to find a way that this code compile:
template < char ... RHS, unsigned int i>
struct t {
static const char s[] = t<' ', char(i+'0'), RHS, i-1>::s;
};
template <char ... RHS >
struct t<RHS, 0> {
static const char s[] = {'0', RHS, '\0'};
};
void main() {
std::cout << t<5>::s; // {'0',' ','1',' ','2',' ','3',' ','4',' ','5','\0'}
}
i from another post, that i dont have the link, but this code is tryng to parser a number to char at compile time. any help why this code is not compiling?
thx in advance!

Answer 1

#include <iostream>
// template parameter pack needs to at the end
template < unsigned int i, char ... RHS >
struct t {
// can't copy-initialize an array from another array
constexpr static char const* s()
{ return t<i-1, ' ', char(i+'0'), RHS...>::s(); };
};
template <char ... RHS >
struct t<0, RHS...> {
// can't initialize a const array inside the class,
// need to use `constexpr`
constexpr static char arr[] = {'0', RHS..., '\0'};
constexpr static char const* s()
{ return arr; }
};
// need to define the array variable, it's ODR-used
template <char ... RHS >
constexpr char t<0, RHS...>::arr[];
int main() {
std::cout << t<5>::s(); // {'0',' ','1',' ','2',' ','3',' ','4',' ','5','\0'}
}
And here's a version with "minimal changes":
#include <iostream>
#include <array>
template < unsigned int i, char ... RHS >
struct t {
constexpr static std::array<char, sizeof...(RHS)+2*i+2> s
= t<i-1, ' ', char(i+'0'), RHS...>::s;
};
template < unsigned int i, char ... RHS >
constexpr std::array<char, sizeof...(RHS)+2*i+2> t<i, RHS...>::s;
template <char ... RHS >
struct t<0, RHS...> {
constexpr static std::array<char, sizeof...(RHS)+2> s
= {{'0', RHS..., '\0'}};
};
template <char ... RHS >
constexpr std::array<char, sizeof...(RHS)+2>
t<0, RHS...>::s;
int main() {
std::cout << t<5>::s.data();
}
Note how the array is copied into each class. The most-derived ("top-level") array is odr-used via .data(), so definition of s for the primary template is necessary. The definition of s for the specialization isn't required here.
Instead of using a static data member, you could also construct the array inside a constexpr function:
constexpr static std::array<char, sizeof...(RHS)+2> arr()
{ return {{'0', RHS..., '\0'}}; }
The drawback is that this returned array has automatic lifetime, so you can't pass its .data() to the base classes.

Answer 2

Here is something similar that will create a string. For example, Stringer<7> will create the string "0 1 2 3 4 5 6 7".
template <uint32_t i>
struct Stringer
{
string str = Stringer<i - 1>().str + " " + to_string(i);
};
template <>
struct Stringer<0>
{
string str = "0";
};
int main(int argc, const char *argv[])
{
cout << Stringer<7>().str << endl;
return 0;
}