C++ Example : Not able to understand what is being done?

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I found this link at http://www.parashift.com/c++-faq-lite/istream-and-ignore.html which shows "How can I get std::cin to skip invalid input characters?" ``` Use std::cin.clear() and std::cin.ignore(). #include #include int main() { int age = 0; while ((std::cout << "How old are you? ") && !(std::cin >> age)) { std::cout << "That's not a number; "; std::cin.clear(); std::cin.ignore(std::numeric_limits::max(), ' '); } std::cout << "You are " << age << " years old "; ... } Of course you can also print the error message when the input is out of range. For example, if you wanted the age to be between 1 and 200, you could change the while loop to: ... while ((std::cout << "How old are you? ") && (!(std::cin >> age) || age < 1 || age > 200)) { std::cout << "That's not a number between 1 and 200; "; std::cin.clear(); std::cin.ignore(std::numeric_limits::max(), ' '); } ... Here's a sample run: How old are you? foo That's not a number be…

Accepted Answer

The insertion and extraction operators `<<` and `>>` on streams will return a reference to the stream itself. That's why you can string together insertions like so: ``` std::cout << "Hello, " << "World!"; ``` First `std::cout << "Hello, "` returns a reference to `std::cout` and then you have the equivalent of `std::cout << "World!"`. The streams can also be converted to `bool`. This basically checks that the state of the stream is still okay. Nothing has failed. For example, you could do this: ``` if (std::cin) // ... ``` This would check if the `std::cin` stream is still in a good state. Now let's look at the code you asked about: ``` while ((std::cout << "How old are you? ") && !(std::cin >> age)) ``` The insertion into `std::cout` won't cause a failure. The only reason it's included here is so that it occurs every time before the input to `age`. The alternative would be to place it once before the `while` and once at the end of the `while` body. After the insertion to `std::cout` is done, `!(std::cin >> age)` will be evaluated. This will first get the user to provide an `age`. Then two things can happen: If this fails in some way (perhaps the user enters characters rather than an integer), then the fail bit will be set. This means that the result of `std::cin >> age` will be converted to a `bool` which will be `false`. The `!` will invert it to `true`. So because both the first and second condition were true, the body of the `while` loop will execute, telling the user that they entered an invalid value and the loop will iterate round again. if the input succeeds, the result of `std::cin >> age` will be `true` and `!` will turn it to `false`. This means the body of the `while` loop will not execute and will not tell the user that they entered the incorrect value. Now let's look at: ``` std::cin.clear(); std::cin.ignore(std::numeric_limits::max(), ' '); ``` This only happens in case 1 above, when the input failed. If a stream goes into a failure state, it will not accept any more insertions or extractions until the state has been cleared. that's what `clear()` does. Then the call to `ignore` says to extract all characters up to and including the next ` ` in the stream and discard them. This gets rid of the invalid input from the stream.

C++ Example : Not able to understand what is being done?

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