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std::cin, std::wcin

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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 constructed before the first constructor of a static object is called and they are guaranteed to outlive the last destructor of a static object, so that it is always possible to read from std::cin in user code.

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.

[edit] Example

#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

[edit] See also

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