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[英]Why does std::when_any use std::tuple rather than std::vector as its result type?
[英]Why does `std::pair` take `std::tuple` as ctor argument type rather than `const std::tuple&`?
C ++ 11標准定義了std::pair
的構造函數,如下所示:
template<class... Args1, class... Args2>
pair(std::piecewise_construct_t, std::tuple<Args1...> first_args,
std::tuple<Args2...> second_args);
為什么std::pair
將std::tuple
用作ctor參數類型而不是const std::tuple&
?
如果復制first_args和second_args很麻煩怎么辦?
目的是使這些元組成為引用的元組,如std::forward_as_tuple
創建的:
Foo x;
Bar y(1, 2, true);
std::pair<A, B> p(std::piecewise_construct,
std::forward_as_tuple(10, x, make()),
std::forward_as_tuple(std::move(y), false, get(), 'a'));
這就像使用A(10, x, make())
等構造對元素。
實際的元組類型為std::tuple<int &&, Foo &, Z &&>
等,它們是輕量級的。
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