char array initialization using cin.get()

Question

I am trying something different with cin.get() like given below:

char chArray[30]="character array with size "; //current string size is 25 character with null character
cout<< chArray << sizeof(chArray)<< endl;
cout<< "Now we will try to enter more than 30 character in chArray using cin.get()";
cin.get(chArray,100);
cout<< chArray << endl << sizeof(chArray)<< endl;

output of above code is very strange as given below:

character array with size 30

Now we will try to enter more than 30 character in chArray using cin.get().

The character array size is 30 but we are entering more than 30 using cin.get() but the size is still 30.

How is size of chArray not changing from 30 to the size of the string we entered using cin.get()?

Please explain.

Answer 1

A fixed array is not dynamically sizable. Once the array is declared, it cannot change size (sizeof() is fixed at compile-time). Your code has a buffer overflow that will corrupt surrounding memory if you try to enter more characters than the array can hold. In your example, your array can only hold 30 chars max, but you are telling cin that it can read up to 100 chars (well, 99, plus a null terminator) into the array.

For what you are trying to do, you need to read into a std::string instead of a char[] array. The size() of a std::string can change dynamically at runtime, eg:

#include <string>

std::string str = "character string with size ";
std::cout << str << str.size() << std::endl;
std::cout << "Now we will try to enter more than 30 character in str using cin";
std::cin >> str; // or: std::getline(std::cin, str);
std::cout << str << std::endl << str.size() << std::endl;

Answer 2

How is size of chArray not changing from 30 to the size of the string we entered using cin.get()?

Arrays in C++ have fixed size. They are created on the stack with a fixed size given by the programmer. That means you give them a specific size and it is known to the compiler at compile time. This size does not change. Ever.

If you write more characters into the array than the size for example writing 100 characters in an array of size 30, it is called buffer overflow or buffer overrrun. It basically means you crossed the boundary i.e., the fixed size set, which is 30 in this case.

The other characters entered (after the limit of 30) can go anywhere in the memory because it is undefined where they will go. If you try to print this array, your program will terminate with an error:

*** stack smashing detected ***: terminated

The error in this particular case means you tried to put more data into the stack than it's capacity.

However, we have string in C++, which you can use if you want a container which changes its size as required. Example:

std::string mystr;
std::cout << "Mystr size before: " << mystr.size() << '\n';
std::getline (std::cin, mystr);
std::cout << "Mystr size after: " << mystr.size() << '\n';