std::ios_base::Init

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< cpp‎ | io‎ | ios base
 
 
 
 
class Init;

This class is used to ensure that the default C++ streams (std::cin, std::cout, etc.) are properly initialized and destructed. The class tracks how many instances of it are created and initializes the C++ streams when the first instance is constructed as well as flushes the output streams when the last instance is destructed.

The header <iostream> behaves as if it defines (directly or indirectly) an instance of std::ios_base::Init with static storage duration: this makes it safe to access the standard I/O streams in the constructors and destructors of static objects with ordered initialization (as long as <iostream> is included in the translation unit before these objects were defined).

Each C++ library module in a hosted implementation behaves as if it contains an interface unit that defines an unexported std::ios_base::Init variable with ordered initialization.

As a result, the definition of that variable is appearance-ordered before any declaration following the point of importation of a C++ library module. Whether such a definition exists is unobservable by a program that does not reference any of the standard iostream objects.

(since C++23)

Member functions

(constructor)
initializes the default C++ streams if they have not been created yet
(public member function)
(destructor)
flushes the default C++ streams if *this is the last instance to be destroyed
(public member function)

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 1123 C++98 the behaviors of the constructor and the destructor
depend on an exposition-only static data member init_cnt
removed the dependency

See also

reads from the standard C input stream stdin
(global object)
writes to the standard C output stream stdout
(global object)
writes to the standard C error stream stderr, unbuffered
(global object)
writes to the standard C error stream stderr
(global object)