Code doesn't work as intended in visual c++ (examples from bjarne stroustrup programming and principles book 2n version)

Question

Okey as I've recently started to read about C++ and try to go with the book I'm having Programming Principles and practice Using C++ 2nd version.

I'm a total newbie so this is probably why, but here goes.

Okey so in the book they have you implement a header .h file instead of the (iostream) etc. So it just have all those for the start as the book doesn't want us to focus on those libraries in the start of the learning. So i implemented it and used it (not sure if this is related to the problem). Anyway at page 77, I'm getting stuck.

Basically it's a wrong value that's getting entered and it's supposed to just show -1(or 0) as the int gets a false value, etc Carlos(letters, not an integer) so the int doesn't get a correct value so the code that is supposed to work (and show 0 or -1 as it's an incorrect value that's entered) is this according to the book:

#include "std_lib_facilities.h"


using namespace std;

int main()
{

cout << "Please enter your first name and age\n";
string first_name = "???"; // string variable // ("???” means “don’t know the name”)
int age = –1; // integer variable (–1 means “don’t know the age”)
cin >> first_name >> age; // read a string followed by an integer
cout << "Hello, " << first_name << " (age " << age << ")\n";
}

and this is what i wrote:

#include "std_lib_facilities.h"


using namespace std;

int main()
{
cout << "Please enter your first name and age" << endl;
string First_Name = "???";
int Age = -1;
cin >> First_Name >> Age;
cout << "Hello, " << First_Name << "(age" << Age << ")" << endl;
keep_window_open();
return 0;
}

However, the result with visual c++ for me is a crash when i for example write 22 Carlos. According to the book it's supposed to output Hello, 22 (age -1)

But for me it just crashes after i enter the word Carlos as value for age... I don't get a a error or anything when i run and compile it, but it crashes after when i give the false value for age.

What am i doing wrong? add: I know i can use a string to get it to work, however I'm just interested in why it doesn't work as I'm following this book i wish to follow it without having these kind of problems as it's intended to work.

Here is a gif when im doing it: http://imgur.com/a/ERjhz Solution: To use system("pause"); instead of keep_window_open(); however it's still annoying to read the book with the knowledge that the code in the book doesn't work always :(

Answer 1

The problem is in keep_window_open. It's a tricky function that is very hard to get absolutely right. You can replace it with this code that sidesteps the problem but only works on Windows:

inline void keep_window_open()
{
   system("pause");
}

or with this slightly more complicated code that tries to take more cases into account:

inline void keep_window_open()
{
    cout << "Please enter a character to exit\n";
    if (!cin)
    {
        cin.clear();
        cin.ignore(120, '\n');
    }
    char ch;
    cin >> skipws >> ch;
}

(This assumes you don't enter lines longer than 120 characters. Replace 120 with numeric_limits<streamsize>::max() if you want it to be free from arbitrary limits. Other functions from std_lib_facilities.h do use 120 in this situation. You many or may not have to add #include <limits> directive to your program).

I have tested in a couple of cases and it appears to work with both correct and incorrect input to the program, but use it at your own risk. It still may or may not work correctly if your program requires more complicated input.

Answer 2

Well it is isn't a problem but too fast to be noticed by our eyes.

i am adding the function definition of keep_window_open()

inline void keep_window_open()
{
    cin.clear();
    cout << "Please enter a character to exit\n";
    char ch;
    cin >> ch;
    return;
}

As you can see it simply takes the character input from the us Forward you will learn about input stream and its buffer So when you input a character in place of integer there is error flagged in the stream(background) and the only one of the characters input is used(in your case 'C' is used for flagging).

I am using input as Carlos 22

So now the input stream is still having characters 'a','r','l','o','s',22 so now the 'a' is used as a input for keep_window_open function and the program ends without waiting for a input from you So there is no error or crash but since the character is already there for input for keep_window_open function so its kind of really fast

Answer 3

A good rule is to always except that the end users is idiots, so you need to write the program in a way that can handles all kinds of inputs without crashing. Therefore always read input numbers as std::string and then try convert it to int, short, double, unsigned int or what ever datatype you are using.

Your code should look like:

#include <cstdlib>
#include <exception>
#include <iostream>
#include <stdexcept>
#include <string>

int main(int argc, char **argv)
{
    int age;
    std::string first_name;
    std::string input_age;

    std::cout << "Please enter your first name and age: " << std::endl;
    std::cin >> first_name >> input_age; // Read the age as 'std::string'

    try
    {
            age = std::stoi(input_age); // Try to convert age from 'std::string' to 'int'
            std::cout << "Hello, " << first_name << " (" << age << ")" << std::endl;
    }
    catch(std::invalid_argument& ex) // 'std::stoi()' will throw 'std::invalid_argument' if it's not able to convert 'std::string' to 'int'
    {
            std::cerr << "Unable to convert input value ('" << input_age << "') to 'int'" << std::endl;
            std::cerr << "Debug info: " << ex.what() << std::endl;
    }
    catch(std::out_of_range& ex) // 'std::stoi()' will throw 'std::out_of_range' if 'std::string' is too small or too big to store in a 'int'
    {
            std::cerr << "The input value (" << input_age << ") is out of range, please input a value inside the range" << std::endl;
            std::cerr << "Debug info: " << ex.what() << std::endl;
    }

    return EXIT_SUCCESS;
}

Answer 4

So it's nothing to do with code, it's something to do with input.

when i for example write 22 Carlos.

The problem is that the code is asking for First_Name then Age, Not Age then First_Name. So when you put 22 for First_Name, the .exe got confused.

for example lets say I did this

int y = 0;
cout << "Give me INT: ";
cin >> y;
cout >> "You put: " >> y;

And when I run the program and put this for input

Give me INT: ghaisewofasd
*crashed*

This is a problem because the user is giving a string for when the code is asking for a int.

So for your case, instead of writing 22 Carlos, just write Carlos 22. Also keep in mind, this book maybe not so correct, because it shouldn't print out Hello, (22). Now a days stuff like that crash if that happens, maybe it was a older version of C++.