main.cpp

#include <Arduino.h>
#include <esp_log.h>

void setup() {
    Serial.begin(115200);
    String a = "test";
    String b = "12345678";
    String out = a + b;
    log_printf("%s + %s = %s\n", a, b, out);

    for (int i = 0; i < out.length(); i++) {
      log_printf("%c", out.charAt(i));
    }
}

void loop() {
  // put your main code here, to run repeatedly:
}

// output:
// test + 12345678 = \��?␏
// test12345678