Thread: c++ input stream and cin

Ok, well, some things are unclear to me about how cin works.

If we for example do this:

Code:

#include <cstdlib>
#include <iostream>

using namespace std;

int main()
{
     int answer = 0;
     string s;
     
     cin >> answer;
     cin >> s;
     
     cout << s << endl;
     
     system("PAUSE");
     return EXIT_SUCCESS;
}

And if the input looks like

not_integer

Why wont cin >> s pick it up?

if the first cin >> answer fails, it pushes the first readed character back to input. so the next cin should pick it up.

but this isnt the case. why?

edit: i also came accros information that cin's bad and easily error prone. Should i avoid using it?

I want my programs to be safe ofcourse.

I come from a C background, thats why this is strange to me.

After attempting to read "answer", the stream will have a flag set that indicates an error condition. That flag needs to be cleared before your code can successfully read s.

If you want safe code, a rule of thumb is: after any operation that may fail, check to see if it has, and recover as needed. You are not doing that.

I'll leave it as an exercise to work out who to check the state of the stream, and to correct it. Reading the documentation to learn effective usage of iostreams won't hurt you.

cin.failbit is a constant. It is the constant you set or clear to indicate or remove the fail flag from the stream. You're not using the correct function.

Originally Posted by grumpy

I'll leave it as an exercise to work out who to check the state of the stream, and to correct it. Reading the documentation to learn effective usage of iostreams won't hurt you.

This is great:

C++ Reference

If you click "iostream", scroll down, click "cin", scroll down, click "istream" (sort of circuitous, but it follows the inheritance structure), you'll notice the method you want is just called "fail()". There's a badbit and a failbit, presumably these are bitflags (get it?).

I like an alternative to MSDN, but where is std::array and std::shared_ptr!?!

Originally Posted by Elysia

I like an alternative to MSDN, but where is std::array and std::shared_ptr!?!

Good question, altho apparently these are not part of the current standard. Eg. Blarney Strnostr's book from last year ("Programming C++") does not mention them either.

But thanks for letting me in on std::array, sounds like a better vector

They're part of TR1, and should be part of C++0x.
std::array is better than native C arrays. They are static arrays, and meant as a replacement for the C arrays (integrates better with the standard library and so on). Not a replacement for std::vector.