Taking string as input but dealing with characters individually in C++

Is there is any efficient method of getting a string as input and then performing some operation on its characters individually?

Yes, there is: read std::string as usual (say, with std::getline or the >> operator of an input stream), and then access the individual characters in a loop.

std::string str;
std::getline(std::cin, str);
for (int i = 0 ; i != str.size() ; i++) {
    std::cout << "Code of character " << i << " is " << (int)str[i] << std::endl;
}

First demo on ideone.

Also, after performing operations, can we output the new string (string got after performing operations) instead of outputting the characters individually using a for loop?

Yes, you can: std::string is mutable, meaning that you can change it in place.

std::string str;
std::getline(std::cin, str);
for (int i = 0 ; i != str.size() ; i++) {
    if (!std::isalpha(str[i])) {
        str[i] = '#';
    }
}
std::cout << str << std::endl;

Second demo on ideone.

You can do it like this:

#include <iostream>
#include <string>

using namespace std;

int main()
{
  string in;
  cout << "Input please\n";
  cin >> in;
  if(in.size() >= 5)
    in[5] = 'A';
  cout << in << "\n";
  return 0;
}

Or you can use std::getline(), instead of std::cin.

Output:

Input please
samaras
samarAs

However, are you sure this is the bottleneck of your program? You can check this with some profiling tools, like the one I use.

[EDIT]

Since OP is asking about efficiency, I did some testing. However, you can to take into account the time that user takes to type the input, but since I am the same person, we can assume this is constant.

So, I did modified a bit a code from another answer, like this:

  std::string str;
  cout << "Input please\n";
  std::getline(std::cin, str);
  if (str.size() >= 5) {
    str[5] = '#';
  }

  std::cout << str << "\n";

Output:

Input please
Samaras
Samar#s
It took me 1.04237 seconds.

And with my code, I got

Input please
Samaras
SamarAs
It took me 0.911217 seconds.

Which actually show that they are pretty close and I would say the difference is due to my typing speed.

I did the timings with std::chrono, like the code I have in my pseudo-site.

Basic operation... Some search on the internet could have helped you but here you go...

std::string processStr(const std::string &str) 
{
  for (std::string::iterator it = str.begin(); it != str.end(); ++it)
    // process your string (getting a char is done by dereferencing the iterator
    //  like this: *it
  return (str);
}