How to make rand() and random() give always different values c++

First of all this is my first time posting here so sorry if I made any mistakes with the title/format. Ok, so I have to make a funcion that returns a random character from a string. I tried making it this way:

include <string>
#include "charset.h"
#include <stdlib.h>
#include <time.h>

namespace Components {
      std::string charSet= " 0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ,.;:-_"; 

     char randChar (){   
         srand(time(NULL));
         uint a = charSet.length();
         int m= rand()%a; 
         return charSet[m];
      }

} 
`

And for some reason it always returns the same char, every time I run the code it generates a new random one and just returns that same one everytime until I stop it and run it again. I thought srand(time(NULL)) was suposed to fix that but it keeps happening. I've tried with random() / srandom(time(NULL)) and they give the same result.

Again, sorry if I made anything wrong or this was alreayd posted somewhere, thanks for any help!