C++ string != string

Question

I have just started C++ and have some experience with C# so I have some programming experience in general. However, seems like straight away I have been shot down. I have tried looking on Google so not to waste anyone's time to no avail.

int main(int argc, char *argv[])
{
    HANDLE  hConsole;
    int k = 5;
    string h;
    string password = "pass";

    hConsole = GetStdHandle(STD_OUTPUT_HANDLE);
    SetConsoleTextAttribute(hConsole, k);
    SetConsoleTextAttribute( GetStdHandle( STD_INPUT_HANDLE ), 0x5B );

    while (h != password)
    {
        printf("This PC is locked\nEnter the password to gain access\n");
        scanf("%s", &h);
    }

    printf("\n");
    system("PAUSE");
    return EXIT_SUCCESS;
}

Whenever I run this it will get me to enter the password and when I click enter it will acknowledge and then crash asking me to either debug or send information to Microsoft. This started when I added the while loop checking two strings. Have I executed this correctly or have I missed something?

Just in case its not clear. I want the program to compare a string with an input and if they are both the same the program will end.

Thanks for looking.

Don't use `scanf` for C strings, and especially not for `std::string`. Use `std::cin` (combined with `std::getline` if needed). — chris, Oct 16 '13 at 18:49

score 5 · Accepted Answer · 2013-10-16T19:00:15.767

5

You simply cannot use scanf() to read into std::string. This is because scanf() is part of C API and it does not support non-POD types. There, %s format specifier expects a pointer to character array with size enough to hold the value (which you cannot really guarantee and that's why scanf() is even considered harmful) whereas you pass it a pointer to object of type std::string (which is far, far different from a plain array). A good compiler would even issue a warning in this case (if not an error, because non-POD types cannot be passed through va_list). Why don't you read into a character array (the C way), or use std::cin instead like std::cin >> h;?

edited Oct 16 '13 at 19:00

answered Oct 16 '13 at 18:51

2

Funny thing is I got that off of a video tutorial - just proves the internet can be the worst source as well as the best. – Marshal Oct 16 '13 at 18:53
3

@Marshal: True. Take a look at [The Definitive C++ Book Guide and List](http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list/). You might find something useful there. – Oct 16 '13 at 18:55
1

@Marshal, Just curious, which tutorial? – chris Oct 16 '13 at 19:32

score 2 · Answer 2 · answered Oct 16 '13 at 18:56

As chris suggested, this program can be simplified by using the correct functions:

#include <iostream>
#include <string>

int main()
{
    std::string password = "pass", input;

    do
    {
        std::cout << "This computer is locked."
                  << "Enter the password to gain access: ";
        std::cin >> input;
    } while (input != password);

    std::cout << "Welcome!" << std::endl;
    system("PAUSE");

    return 0;
}

score 0 · Answer 3 · answered Oct 16 '13 at 18:56

0

1/ As many beginners in C++, you are actually writing C code. Look for documentation about input/ouput in C++.

2/ How do you compile your code? Your error should have been caught by the compiler

answered Oct 16 '13 at 18:56

vincentB

128
1
8

Currently using DevC++ - the free C++ IDE and press F9 to compile and run the program. – Marshal Oct 16 '13 at 18:58
1

@vincentB The compiler can't detect that a wrong type's address is passed to `scanf()`! The program will compile, run and crash as the OP described. – πάντα ῥεῖ Oct 16 '13 at 19:01
At least VS2010 compiler is warning me: warning C4996: 'scanf': This function or variable may be unsafe. – vincentB Oct 16 '13 at 19:39

C++ string != string

3 Answers3