snprintf to pre-sized std::string doesn't work?

Question

I'm a bit lost why Option 1 doesn't work. In my understanding I should be able to write to a presized std::string whatever I want as long as I don't overshoot the string size, but it doesn't work on godbolt and on my ESP32 it even causes a heap corruption downstream. What's the issue?

LiveDemo

#include <iostream>
#include <vector>
#include <string>
#include <cstdint>


#define MAC2STR(a) (a)[0], (a)[1], (a)[2], (a)[3], (a)[4], (a)[5]
#define MACSTR_UPPER              "%02X:%02X:%02X:%02X:%02X:%02X"


// Option 1: Doesn't work

// auto get_mac_address() -> std::string {
//      const std::size_t s = 17;
//      auto ret = std::string{ s, '\0' };
//      uint8_t int_mac_addr[6] = { 0xFF, 0x16, 0x01, 0x25, 0xBB, 0x7A };
//      std::snprintf(&ret.front(), s+1, MACSTR_UPPER, MAC2STR(int_mac_addr));
//      return ret;
// }


// Option 2 Works? Why?

auto get_mac_address() -> std::string {
    uint8_t int_mac_addr[6] = { 0xFF, 0x16, 0x01, 0x25, 0xBB, 0x7A };
    char mac_str[18]; // 17 characters for MAC address + 1 for null terminator
    std::snprintf(mac_str, sizeof(mac_str), MACSTR_UPPER, MAC2STR(int_mac_addr));
    return std::string(mac_str);
}
 

int main() {
    std::vector<std::string> vec = { std::string("my/mqtt/") + get_mac_address() + "/#",
                                    std::string("other/mqtt/") + get_mac_address() + "/#"};

    for (auto v : vec) {
       std::cout << v << std::endl; 
    }
}

Answer 1

You are using the std::string constructor taking a std::initializer_list<char> so auto ret = std::string{ s, '\0' }; makes ret a string with 2 characters only.

Use...

auto ret = std::string(s, '\0');

...instead.