std::cin.getline( ) vs. std::cin

问题

When should std::cin.getline() be used? What does it differ from std::cin?

回答1:

In case with char*, std::cin.getline getting line, instead of std::cin getting first word.

回答2:

Let's take std::cin.getline() apart. First, there's std::. This is the namespace in which the standard library lives. It has hundreds of types, functions and objects.

std::cin is such an object. It's the standard character input object, defined in <iostream>. It has some methods of its own, but you can also use it with many free functions. Most of these methods and functions are ways to get one or more characters from the standard input.

Finally, .getline() is one such method of std::cin (and other similar objects). You tell it how many characters it should get from the object on its left side (std::cin here), and where to put those characters. The precise number of characters can vary: .getline() will stop in three cases: 1. The end of a line is reached 2. There are no characters left in the input (doesn't happen normally on std::cin as you can keep typing) 3. The maximum number of characters is read.

There are other methods and functions that can be used with the std::cin object, e.g.

  std::string s;
  int i;
  std::cin >> s; // Read a single word from std::cin
  std::cin >> i; // Read a single number from std::cin
  std::getline(std::cin, s); // Read an entire line (up to \n) from std::cin
  std::cin.ignore(100); // Ignore the next 100 characters of std::cin

回答3:

Did you read any documentation (e.g. http://www.cplusplus.com/reference/string/getline/)?

Basically, std::cin (or more generally, any std::istream) is used directly in order to obtain formatted input, e.g. int x; std::cin >> x;. std::cin.getline() is used simply to fill a raw char * buffer.

回答4:

As already others have answered (even better) roughly speaking, use getline() to read an entire line (i.e., a string terminating with \n) and cin>>var to read a number compatible with the type of var (integer, float, double etc.) or a single word.

In this answer I want to emphasize a problem that arises when mixing the two methods. When you do:

int a;
string s;
cin>>a;
getline(cin, s)

cin leaves an end of line, \n, character which is then read by getline();. It is possible to overcome this problem by using cin.ignore().

int a;
string s;
cin>>a;
cin.ignore();
getline(cin, s)

来源：https://stackoverflow.com/questions/4745858/stdcin-getline-vs-stdcin