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# std::once_flag

From cppreference.com
< cpp‎ | thread

Defined in header `<mutex>` |  |
class once_flag; |  | (since C++11)

The class `std::once_flag` is a helper structure for std::call_once.

An object of type `std::once_flag` that is passed to multiple calls to std::call_once allows those calls to coordinate with each other such that only one of the calls will actually run to completion.

`std::once_flag` is noncopyable.

### [edit] Member functions

# std::once_flag::once_flag

once_flag(); |  |

Constructs an `once_flag` object. The internal state is set to indicate that no function has been called yet.

### Parameters

(none)

### Exceptions

`noexcept` specification:
noexcept

### [edit] See also

call_once (C++11) | invokes a function only once even if called from multiple threads (function template) [edit]

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Category:

- unconditionally noexcept