What is the difference between following two string cases-CodePudding

I am trying to assign the value to string and then access it, but for the 1st case i didnt get output as wanted....

#include <iostream>
#include <string>
using namespace std;
 
int main(){
      
      string abc;

      abc[0] = 'm';

      cout << "str1 : " << abc    << endl;
      cout << "str1 : " << abc[0] << endl;

      //-----------------------------------

      string xyz;

      xyz = "village";

      cout << "str2 : " << xyz    << endl;
      cout << "str2 : " << xyz[0] << endl;

      return 0;
}

Output Should Be :

str1 : m
str1 : m
str2 : village
str2 : v

But Actually It Is :

str1 :
str1 : m
str2 : village
str2 : v

CodePudding user response：

In the first case you are replacing the null-terminator (which each string has at end, to mark end), with m, and probably causing a lot of random output, if not SIGSEG crash and/or undefined behavior.

Solutions

Array style

std::string myVariable;

// ...

myVariable.resize(3, '\x0');
myVariable[0] = 'm';
myVariable[1] = 'a';
myVariable[2] = 'x';
// And index 3 is already null-terminator (no need to set manually to zero).

What you should do instead

If speed is not a concern, and you just want stability and ease of use, try something like:

abc  = 'm';

abc.append(1, 'm');

CodePudding user response：

There is a mistake in your program as described below.

Mistake

string abc; //this creates a **0 sized** default constructed string object
abc[0] = 'm';// incorrect because you're trying to assign to the 0th index of the string object abc but note that there is no 0th index because the string has 0 size.

Note that the statement

abc[0] = 'm';

is incorrect because at the moment the string object abc has 0 size and you're trying to access the 1st element(which have 0th index). But wait, how can you access the 1st element(or 0th index) of a string that doesn't have any element because it is of size 0.

Solution 1

Instead of assigning to the 0th index you can append to the string abc using

abc  = 'm';

Solution 2

You can also create a std::string with a particular size(length) and element as follows:

std::string abc(1, 'm'); //now abc has size(length) of 1 and has the element(0th element) as "m".

with this you don't need to use the assignment you're doing. That is now you don't need abc[0] = 'm';

CodePudding user response：

Your code has undefined behavior. After string abc; abc is empty and contains no elements. abc[0] = 'm'; is trying to perform assignment on abc's 1st element which doesn't exist.

You might want

abc  = 'm'; // append 'm' to abc

abc.append(1, 'm'); // append 'm' to abc