std::cin, std::wcin

From cppreference.com
< cpplrm; | io
C++
Input/output library
std::basic_istream
Defined in header <iostream>
extern std::istream cin;
 (1)
extern std::wistream wcin;
 (2)

The global objects std::cin and std::wcin control input from a stream buffer of implementation-defined type (derived from std::streambuf), associated with the standard C input stream stdin.

These objects are guaranteed to be initialized during or before the first time an object of type std::ios_base::Init is constructed and are available for use in the constructors and destructors of static objects with ordered initialization (as long as <iostream> is included before the object is defined).

Unless sync_with_stdio(false) has been issued, it is safe to concurrently access these objects from multiple threads for both formatted and unformatted input.

Once std::cin is constructed, std::cin.tie() returns &std::cout, and likewise, std::wcin.tie() returns &std::wcout. This means that any formatted input operation on std::cin forces a call to std::cout.flush() if any characters are pending for output.

Notes

The 'c' in the name refers to "character" (stroustrup.com FAQ); cin means "character input" and wcin means "wide character input"

Example

Run this code
#include <iostream>
struct Foo {
    int n;
    Foo() {
       std::cout << "Enter n: "; // no flush needed
       std::cin >> n;
    }
};
Foo f; // static object
int main()
{
    std::cout << "f.n is " << f.n << '\n';
}

Output: 

Enter n: 10
f.n is 10

See also

 initializes standard stream objects
(public member class of std::ios_base)
writes to the standard C output stream stdout
(global object)
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