Initial Check to see if a Substring is within range - c++

Question

I am in the beginnings of learning C++ and I am wondering if there is a way to assert that a substring can be created from a String, given a range. My String will vary in size each iteration. I am trying to create six substrings from that original String. With this variation in size, I am sometimes trying to access indexes of the String that do not exist for that particular iteration.
For example, if my String in iteration 1 is 11 characters
My first substring is from 3 characters - valid
My second substring is the next 3 characters - valid
My third substring is the next 5 characters - valid
My fourth substring is the next 4 characters - not valid - crashes program
My fifth substring - not valid, out of range
My sixth substring - not valid, out of range
I am wondering if there is a small check I can do to assert the length is valid. It's worth noting, I suppose, that I have not set any default values to these substrings. They are declared as:
string subS1
string subS2
string subS3
...
...
string subS6
Would setting all 6 substrings to null upon declaration alleviate this issue and for any valid substring, the value will just be overwritten?
Thanks in advance
subS1 = str.substr(0, 3); // Could be valid range
subS2 = str.substr(3, 3); // Could be valid range
subS3 = str.substr(6, 5); // Could be valid range
subS4 = str.substr(11, 4); // Could be valid range
subS5 = str.substr(15, 4); // Could be valid range
subS6 = str.substr(19); // from the nineteenth character to the end

Answer 1

Algorithm--->
step 1: Get the length of string in current iteration in variable size.
step 2: Write this code in itertaion.
int i=0;
i= str.substr(start,end).length();
if( i>size) then,
std::cout<<"index exceeded";

Answer 2

Either check the size of str before extracting the string, or rely on std::string::substr's len parameter:
Number of characters to include in the substring (if the string is
shorter, as many characters as possible are used). A value of
string::npos indicates all characters until the end of the string.
Example:
#include <iostream>
#include <string>
int main ()
{
std::string str="Alexander the Great";
std::string str2 = str.substr (16, 25);
std::cout << str2 << '\n'; // eat
return 0;
}
It won't crash, it will just use as many characters as possible.
This however:
std::string str2 = str.substr (20, 25);
should crash, so do it like this in this case:
std::string str2 = ""; // initialise to empty string, that is the NULL you are saying I guess
if(str.size() > 20)
str2 = str.substr (20, 25);
// 'str2' will be an empty string