char pointer working in scanf but not in cin.

Hi
This seems very easy and beginner question.But really i can't figure out it.

Code:

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  char *ch
    cout << "Enter name";
  scanf("%s",&ch);
    cin >> ch;

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Why can not cin allocate ch pointer but scanf can allocate it.What is happening?It gives run-time error.
I am looking for your answers.

Scanf doesn't allocate pointers either. You maybe getting lucky. Always allocate/init pointers!

As far as I can tell, both dont work (syntax errors aside). You havent allocated any memory for the null terminated string ch.

hmm yes you are right.Also i see that scanf didn't work if i write:
cout >> ch;

What are the differneces between string and char pointer.I mean this code is working :

Code:

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string s;
    cout << "Enter name";
  cin >> s;
  cout  << s;

---

but if declare s char pointer, it doesn't.We know that s is a pointer isn't it?Also ch is a pointer.But still i don't understand the differences..AlsoIs there a way that i can assign variable size string to char pointer.How can string variable achieve this?
Please explain.
Thanks for answers.

Quote:

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We know that s is a pointer isn't it?

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No, s is a std::string object.

Quote:

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Is there a way that i can assign variable size string to char pointer

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Yes, by using the c_str() member function of the std::string object. If you want this pointer to a null terminated string of characters to take ownership of the string contents, then you should copy the contents over, since the std::string retains ownership of what is returned by c_str().

If you want to do this by char pointer then allocate the required memory through new operator.

Code:

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char *a=new char[20];
cin>>a;
cout<<a;

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But be sure you don't enter more than 19 word string (1 reserved for NULL character)

Quote:

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Originally Posted by vaibhav

If you want to do this by char pointer then allocate the required memory through new operator.

Code:

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char *a=new char[20];
cin>>a;
cout<<a;

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But be sure you don't enter more than 19 word string (1 reserved for NULL character)

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Interesting. You're not supposed to use cin.get() for this anymore?
And what is wrong with std::string? Even if you really wanted a C-style version stored somewhere that's easy enough to do.

Code:

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text[] = s.c_str()

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What does that do? I get a compilation error, maybe strcpy is needed?

>> char *text = new char[ s.size() ];
>> text = s.c_str()

That code is wrong citizen, you are assigning the pointer returned by c_str and leaking the memory allocated for text. Use strcpy. Also, you need to allocate space for the null terminator, so you should use new char[ s.size() + 1];.

It stores the c-style string in text. Yeah, I didn't compile anything before I posted that. And .c_str() returns a const char, and I missed a semi-colon. I fixed it though.

Quote:

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Originally Posted by vaibhav

If you want to do this by char pointer then allocate the required memory through new operator.

Code:

---

char *a=new char[20];
cin>>a;
cout<<a;

---

But be sure you don't enter more than 19 word string (1 reserved for NULL character)

---

No.
I want to do this without giving size.
I mean if we use string variable.We didn't give size before use.And string can allocate memory whatever its size at runtime.For example you said we must enter 19 character.But if a was a string, its size is not important.

String is not a variable type.It uses char pointers.I want to understand how string can achieve this. I hope i can explain what my problem is.

Thanks for answers.

string is a class that defines a type. There are different ways for it to be implemented, but it generally has a dynamic array of characters internally that it uses to store the string. If you have a variable of type string (like your s above), then it has overloaded the operator>> to read in characters from the input stream like cin.

Quote:

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Originally Posted by sawer

No.
I want to do this without giving size.
I mean if we use string variable.We didn't give size before use.And string can allocate memory whatever its size at runtime.For example you said we must enter 19 character.But if a was a string, its size is not important.

String is not a variable type.It uses char pointers.I want to understand how string can achieve this. I hope i can explain what my problem is.

Thanks for answers.

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std::string is a variable type. The implementation is unimportant.

If you don't want to worry about giving the size of the string before stroring it, that's what std::string is for.

If you really want to make things dificult for yourself by using C strings, you can create a function or an object that will store put to memmory undefined length string to a character array. After all that's what std::string does internally, and it's written in C++. I don't actually know how std::string manages it, but vectors allow simmilar functionality by allocating a huge amount of space, and realocating more if your data does not fit. I suspect std::string works the same way.

A common solution is to double the amount of data allocated whenever necessary. This will not use up too much memory, but is stll very fast.

std::string will automatically resize to fit any string you pass it, at least until you run out of RAM.

ok.
Thanks friends.
I see that string doesn't use only one pointer to reallocate memory while user enter characters.

Thanks.
Good works...