std::ignore
Defined in header <tuple> |
||
---|---|---|
const /*unspecified*/ ignore; |
(since C++11) (until C++17) |
|
inline constexpr /*unspecified*/ ignore; |
(since C++17) |
An object of unspecified type such that any value can be assigned to it with no effect. Intended for use with std::tie
when unpacking a std::tuple
, as a placeholder for the arguments that are not used.
It can also be used to avoid warnings from unused return values of [[nodiscard]]
functions.
Possible implementation
namespace detail { struct ignore_t { template <typename T> constexpr // required since C++14 void operator=(T&&) const noexcept {} }; } inline constexpr detail::ignore_t ignore; // 'const' only until C++17 |
Example
- Demonstrates the use of
std::ignore
together with a[[nodiscard]]
function. - Unpacks a
std::pair<iterator, bool>
returned byset.insert()
, but only saves the boolean.
#include <iostream> #include <string> #include <set> #include <tuple> [[nodiscard]] int dontIgnoreMe() { return 42; } int main() { std::ignore = dontIgnoreMe(); std::set<std::string> set_of_str; bool inserted = false; std::tie(std::ignore, inserted) = set_of_str.insert("Test"); if (inserted) { std::cout << "Value was inserted successfully\n"; } }
Output:
Value was inserted successfully
See also
(C++11)
|
creates a tuple of lvalue references or unpacks a tuple into individual objects (function template) |
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