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Showing content from https://cplusplus.com/reference/future/future_errc/ below:

enum class

std::future_errc

Error conditions for future objects

This enum class type defines the error conditions of the future category.

future_errc label int value description broken_promise 0 The promise object with which the future shares its shared state was destroyed before being set a value or an exception. future_already_retrieved 1 A future object was already retrieved from this provider. promise_already_satisfied 2 The promise object was already set a value or exception. no_state 3 An operation attempted to access the shared state of an object with no shared state.

All library implementations define at least the values above, but may provide additional values.

future_errc label int value description broken_promise * The promise object with which the future shares its shared state was destroyed before being set a value or an exception. future_already_retrieved * A future object was already retrieved from this provider. promise_already_satisfied * The promise object was already set a value or exception. no_state * An operation attempted to access the shared state of an object with no shared state.

* = A value different from zero (including

broken_promise

). The particular values may vary according to library implementation, but these four values are guaranteed to exist and be distinct. Implementations may provide additional labels and values.

Values of the enum type future_errc may be used to create error_condition objects to be compared against the value returned by the code member of future_error.

Non-member overloaded functions

make_error_code (future_errc): Make error code (function)

make_error_condition (future_errc): Make error condition (function)

Non-member class specializations

is_error_code_enum (future_errc): error_code enum flag for future_errc (class)

Example

// std::future_errc example:
#include <iostream>     // std::cerr
#include <future>       // std::promise, std::future_error, std::future_errc

int main ()
{
  std::promise<int> prom;

  try {
    prom.get_future();
    prom.get_future();   // throws std::future_error with future_already_retrieved
  }
  catch (std::future_error& e) {
    if (e.code() == std::make_error_condition(std::future_errc::future_already_retrieved))
    std::cerr << "[future already retrieved]\n";
    else std::cerr << "[unknown exception]\n";
  }

  return 0;
}

Output (on stderr):

[future already retrieved]