C++ :: Char Pointers And Random Binary Files

I'm new with working with random binary files. I have a class with a char* pointer stored inside of it, I also have a constructor that takes in a string (of any size) from the user. I then simply store this string into the char *. Once the string is stored in the char *. I reinterpret the instance, and I store the information into the random binary file. Everything works.

Question: Random files must know the size of the object that is being stored inside of it, so why when I enter strings of different sizes into the file, it appears to still be working. for example this is an example of the code:

class info {
private:
char *phrase;
public:
info(string n ="unknown"){
phrase = new char[n.size()+1];

[Code] ....

My point is, lets just say for example the object ETC, was some long string, this would still work for me. My question is, I don't believe each object is the same size because I allocate memory for the char pointer in the constructor.

Should I not do this just to be safe, and just use a char array instead of a pointer? (Even tho I would have set a pre-defined size for the string) or is something happening in the back to prevent this from not working?