How to calculate the length of output that sprintf will generate?

Question

Goal: serialize data to JSON.

Issue: i cant know beforehand how many chars long the integer is.

i thought a good way to do this

Answer 1

Calling snprintf(nullptr, 0, ...) does return the size but it has performance penalty, because it will call IO_str_overflow and which is slow.

If you do care about performance, you can pre-allocate a dummy buffer and pass its pointer and size to ::snprintf. it will be several times faster than the nullptr version.

template
size_t get_len(const char* format, Args ...args) {
  static char dummy[4096]; // you can change the default size
  return ::snprintf(dummy, 4096, format, args...) + 1; // +1 for \0
}