test.cpp

// Works
class Something {
  public:
    void doSomething() {
      uint8_t buffer[1024];
      
      memcpy(buffer + bufferPosition, something);
      bufferPosition += someSize;
      
      memcpy(buffer + bufferPosition, something);
      bufferPosition += someSize;
      
      bufferPosition = 0;
    }
    
  private:
    uint8_t bufferPosition = 0;
}

// Doesn't work
class Something {
  public:
    void doSomething() {
      memcpy(buffer + bufferPosition, something);
      bufferPosition += someSize;
      
      memcpy(buffer + bufferPosition, something);
      bufferPosition += someSize;
      
      bufferPosition = 0;
    }
    
  private:
    uint8_t buffer[1024];
    uint8_t bufferPosition = 0;
}