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#pragma once
/// @file String view that can be either owned or borrowed.
#include <variant>
#include <string>
#include <string_view>
/**
* This wants to be a little bit like rust's Cow type.
* Some parts of the evaluator benefit greatly from being able to reuse
* existing allocations for strings, but have to be able to also use
* newly allocated storage for values.
*
* We do not define implicit conversions, even with ref qualifiers,
* since those can easily become ambiguous to the reader and can degrade
* into copying behaviour we want to avoid.
*/
class BackedStringView {
private:
std::variant<std::string, std::string_view> data;
/**
* Needed to introduce a temporary since operator-> must return
* a pointer. Without this we'd need to store the view object
* even when we already own a string.
*/
class Ptr {
private:
std::string_view view;
public:
Ptr(std::string_view view): view(view) {}
const std::string_view * operator->() const { return &view; }
};
public:
BackedStringView(std::string && s): data(std::move(s)) {}
BackedStringView(std::string_view sv): data(sv) {}
template<size_t N>
BackedStringView(const char (& lit)[N]): data(std::string_view(lit)) {}
BackedStringView(const BackedStringView &) = delete;
BackedStringView & operator=(const BackedStringView &) = delete;
/**
* We only want move operations defined since the sole purpose of
* this type is to avoid copies.
*/
BackedStringView(BackedStringView && other) = default;
BackedStringView & operator=(BackedStringView && other) = default;
bool isOwned() const
{
return std::holds_alternative<std::string>(data);
}
std::string toOwned() &&
{
return isOwned()
? std::move(std::get<std::string>(data))
: std::string(std::get<std::string_view>(data));
}
std::string_view operator*() const
{
return isOwned()
? std::get<std::string>(data)
: std::get<std::string_view>(data);
}
Ptr operator->() const { return Ptr(**this); }
};
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