Question

#include <iostream>
using namespace std;

void main(){
  char name[20];
  cin>>name;         // when I input "This is" 
  cout<<name<<endl;  // output was "This" 
}

How to do that when I input “This is” , output too will be “This is” , not only “This” ?

Answer 1

You could use

cin.get( name, 20 );

or

cin.getline(name, 20);

Depending on whether you want the new line character in your string.

EDIT:

If you want to further simplify your code you could use a string instead of a char array, you wouldn’t have to worry about the user exceeding your buffer size then ie.

string name;
getline(cin, name);
cout << name << endl;

EDIT 2:

As David Heffernan also pointed out in a comment I should explain why it’s not working for you as expected. The extraction operator (operator >>) ends when a whitespace character — a space in your case — is reached. The operator also terminates when a null-character or the end of file is reached.