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[std-proposals] Add operator>> overloads to std::optional for streaming input

From: Kevin Schmidt <kevin_at_[hidden]>
Date: Sun, 25 Jun 2023 00:37:52 +0200
Hello everybody, I would like to propose the following for the new standard:


Currently, std::optional does not have an operator>> defined, so values
cannot be streamed directly into an std::optional. Users have to explicitly
extract the value from the stream and assign it to the std::optional using
emplace() or operator=(). By adding operator>> overloads, std::optional can
support more intuitive streaming input.

Initial Design Proposition:

template <typename T>
std::istream& operator>>(std::istream& is, std::optional<T>& opt) {
    T value;
    if (is >> value) {
    } else {
    return is;

This defines an operator>> that extracts a value of type T from the input
stream "is" and assigns it to the std::optional<T> object "opt". If the
extraction succeeds, the value is emplace()d into "opt". If it fails, "opt"
is reset to an empty state. This is just a basic implementation that can be
used without the internals. For the implementation into the standard I
would suggest a rewrite using the internals of std::optional to overcome
the issue of using one unnecessary stack allocation. I still believe that
mentioning this is important though since this code can be used as a
premature implementation for now.

The implementation allows code like:

std::optional<int> opt;
std::istringstream("123") >> opt;
// opt is 123

Impact on the Standard:

This change will make std::optional more intuitive to use for streaming
input, as values can be extracted directly into an optional without needing
to handle the empty case explicitly. It brings std::optional inline with
other wrapper types like std::variant that support streaming through
operator>> overloads.

The proposed changes are limited to overloads for std::istream and its
derived types. Overloads for other stream types can be added separately.
The proposed design is limited to a single-argument overload to avoid
ambiguity, but additional overloads can be defined if needed.

This change should have limited impact on existing code, as it purely adds
additional functionality. The proposed design has been kept simple, but can
be extended in the future if needed.

I hope that this fits nicely into the standard, any feedback or improvement
suggestions are welcome.

Thanks for your attention,

Received on 2023-06-24 22:38:05