Convert variadic template arguments to JSON string

Question

I need a function that receives variadic template arguments and returns a JSON array string.

I was able to reach this point:

#include <iostream>
#include <sstream>
#include <string>

using namespace std;

template<typename Arg>
void add_value(std::stringstream &result, Arg arg){

  if (result.rdbuf()->in_avail())
    result << ",";

  if (std::is_same_v<Arg, string>) {
    result << "\"" << arg << "\"";
  } else if (std::is_same_v<Arg, const char*>) {
    result << "\"" << arg << "\"";
  } else if (std::is_same_v<Arg, char*>) {
    result << "\"" << arg << "\"";
  } else if (std::is_same_v<Arg, bool>) {
    result << arg;
  } else {
    result << arg;
  }
}

template<typename... Args>
std::string to_json_array(Args... args) {
  std::stringstream result;
  (add_value(result, args), ...);
  return "[" + result.str() + "]";
}

int main() {
  std::string result;
  char aa[10] = "haha";
  char *bb = aa;

  result = to_json_array(123, "Hello", 1.5, string("World!"), true, aa, false, bb, NULL);

  std::cout << result << "\n";
}

But it did not work for booleans and null.

As you can notice, I am not a C++ developer.

If you know another method, you can share it too.

What does "did not work" mean? We really can't do anything with that level of detail. — Asteroids With Wings, May 18 '20 at 20:00
It returns `1` instead of `true` and `0` instead of `false` and `null` — Bernardo Ramos, May 18 '20 at 20:01
Definitely use a library for that. It's going to be much more complex than what you have. — Yksisarvinen, May 18 '20 at 20:06
Wow, it worked for the booleans. Just lacking the null now. Answering: no, I don't know how nulls are handled in C++ — Bernardo Ramos, May 18 '20 at 20:08
There is no such thing as null object in C++. `NULL` is macro, which expands to `0`. There is no way you can convert that to `"null"` within your function (unless you convert all `0` integer values to `"null"`). You can use `nullptr` (which is of type `std::nullptr_t`) as a modern way substitute, but for actual handling of JSON files this will not do. — Yksisarvinen, May 18 '20 at 20:16
Thanks for the help. I changed it to `nullptr` in my tests, but the checking code does not compile: `if (std::is_same_v)` and `if (is_null_pointer(arg))` — Bernardo Ramos, May 18 '20 at 20:26
You just need [some `constexpr`](https://en.cppreference.com/w/cpp/language/if#Constexpr_If) so the invalid branches don't get compiled (`ostream << arg` is invalid when `arg` is a `nullptr_t`) - https://repl.it/repls/QuietPepperyComputerscience — Asteroids With Wings, May 18 '20 at 21:03

score 0 · Answer 1 · answered May 18 '20 at 21:10

I am sharing the solution I've got with the help of the comments:

#include <iostream>
#include <sstream>
#include <string>

using namespace std;

template<typename Arg>
void add_value(std::stringstream &result, Arg arg){

  if (result.rdbuf()->in_avail())
    result << ",";

  if constexpr (std::is_same_v<Arg, string>) {
    result << "\"" << arg << "\"";
  } else if constexpr (std::is_same_v<Arg, const char*>) {
    result << "\"" << arg << "\"";
  } else if constexpr (std::is_same_v<Arg, char*>) {
    result << "\"" << arg << "\"";
  } else if constexpr (std::is_same_v<Arg, bool>) {
    result << std::boolalpha;
    result << arg;
    result << std::noboolalpha;
  } else if constexpr (std::is_same_v<Arg, nullptr_t>) {
    result << "null";
  } else {
    result << arg;
  }
}

template<typename... Args>
std::string to_json_array(Args... args) {
  std::stringstream result;
  (add_value(result, args), ...);
  return "[" + result.str() + "]";
}

int main() {
  std::string result;
  char aa[10] = "haha";
  char *bb = aa;

  result = to_json_array(123, "Hello", 1.5, string("World!"), true, aa, false, bb, nullptr, 0);

  std::cout << result << "\n";
}

Thank you all!

Convert variadic template arguments to JSON string

1 Answers1