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Optional arguments

1234567891011121314 #include <optional> void foo(int i, std::optional<double> f, std::optional<bool> b) { } int main() { foo(5, 1.0, true); foo(5, std::nullopt, true); foo(5, 1.0, std::nullopt); foo(5, std::nullopt, std::nullopt); }

This pattern is licensed under the CC0 Public Domain Dedication.

Requires c++17 or newer.

Intent

Allow argument values to be omitted when calling a function.

Description

The function foo, on lines 3–6, takes three arguments, two of which have type std::optional (from the Library Fundamentals TS). This allows the value of those arguments to be omitted, as shown on lines 10–13, where std::nullopt represents no value.

This approach is more expressive than using pointers and nullptr. A related technique is the use of default arguments, which allow arguments to be omitted entirely, but only from the end of the argument list.

If you are constructing an object with a complex combinatorial set of optional arguments, consider using the builder pattern.

Contributors

Last Updated

27 August 2018

Source

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