C++ store string input as cstring

0
votes

I am doing an assignment where the first part requires us to prompt the user to enter a string. We need to store this input as a cstring on the heap (not a std::string so we get a better understanding of cstring). I have got something that works fine:

char *input = new char[500];
cout << "enter a string: ";
cin.getline(input, 500);

But I feel like this isn't a very good way to do it in regards to creating an array of 500 and hoping they don't exceed this character limit. Is there a way I can do this without making a ridiculously large array and likely not using anywhere near that much? This seems a very amateurish way of going about it. I think it is possible using std::string but I have to use cstring :(

3
c++getlinecstring
If you don' specify the size then how will the heap know what amount of space it should make for your character? It will assign space for 500 chars here and make sure that this space is not taken over by other apps. - user142650
The way std::string probably does it is by allocating a smallish buffer, then reallocating and copying once this fills up, and possibly repeating and reallocating multiple times until there are no more characters to be read. You could implement it this way, but it's not worth the trouble. 500 bytes is not very much space nowadays. - Brian Bi
std::string would automatically size itself to fit, so obviously it would be a lot easier to use. In general, the way you did it is about how well you can do it without knowing enough about the size of the input to be more precise. If you wanted to save overall space, you could fill your buffer, then find its size and make another array of exactly that size, copy your buffer to that array and then deallocate the buffer. - IllusiveBrian
you'd have to check the failbit and start a new buffer if it is set. std::string probably does something like this. - Red Alert
Use std::string and it's reserve method if you know output could be large. - zoska

3 Answers

1
votes

The C way would be to allocate the buffer with malloc, read the appropriate number of bytes, and figure out if you got the whole string. If you didn't read enough, then you can use realloc to expand the buffer, and read more where you left off.

0
votes

we can do like this:BUILD A DYNAMIC STRING BUFFER YOURSELF.

  1. Alloc a heap memory(80 is enough).
  2. You control the input by yourself.You can read just one character from the input stream every time.If the size of the memory is not enough to contain this character,just alloc another enough one.
  3. When you get the character '\n'(or others) just stop reading.

Yes,this is what the c++ library does!The buffer is actually a queue.

0
votes

c++ improved performance... so this takes an array and points to that array with pointer 

#include <cstring>
#include <iostream>
#include <string>
#include <stdio.h>
using namespace std;

int main()
{
char name[100];
//string userInput[26];
int i=0, n=0;
cout<<"your name? ";
cin>>name;
cout<<"Hello "<<name<< endl;

char *ptr=name;
for (i = 0; i < 20; i++)
{
cout<<i<<" "<<ptr[i]<<" "<<(int)ptr[i]<<endl;
}   
int length = 0;
while(name[length] != '\0')
{
    length++;
    }

            for(n=0; n<3; n++)
                    {

                if (strncmp(ptr, "r2d2", 4) == 0)
                    {
                    cout << "you found r2d2bob" << ptr[i];
                    }

        }
cout<<name <<"is"<<length<<"chars long";
}