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◆ main openmsx / src / utils / static_string_view.hh
2.4 KiB · C++ · last changed chore(): initial commit 1 month ago source blame history raw
e8555c89 Andy McCall 1 
#ifndef STATIC_STRING_VIEW_HH
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#define STATIC_STRING_VIEW_HH
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#include "StringStorage.hh"
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#include <algorithm>
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#include <cassert>
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#include <string_view>
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#include <utility>
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/** static_string_view
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 * This is like std::string_view, but points to a string with static lifetime
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 * (e.g. a string literal). In other words, the pointed-to-string is guaranteed
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 * to remain valid. This allows to store just a reference to the string instead
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 * of having to copy it into e.g. a std::string object.
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 *
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 * The intention is that objects of this class can only be constructed from
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 * string-literals. For example constructing from std::string, std::string_view
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 * or 'const char*' will result in compile errors. (See unittest for more
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 * details).
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 *
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 * In some (rare?) cases you may want something like this, but string literals
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 * are too restrictive. Using the 'make_string_storage()' function below might
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 * be a solution. But then the programmer takes responsibility of correctly
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 * managing lifetimes (then the c++ compiler doesn't help anymore).
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 */
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class static_string_view
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{
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public:
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	// Disallow construction from non-const char array.
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	template<size_t N>
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	constexpr static_string_view(char (&buf)[N]) = delete;
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	// Construct from string-literal (a const char array).
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	template<size_t N>
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	constexpr static_string_view(const char (&buf)[N])
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		: s(buf, N-1)
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	{
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		assert(buf[N - 1] == '\0');
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	}
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	// Used by make_string_storage().
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	struct lifetime_ok_tag {};
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	constexpr static_string_view(lifetime_ok_tag, std::string_view v)
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		: s(v) {}
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	// Allow (implicit) conversion to std::string_view.
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	explicit(false) constexpr operator std::string_view() const { return s; }
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private:
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	std::string_view s;
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};
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/** Take a string_view, make a copy of it, and return a pair of
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  *  - Storage for the copy.
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  *  - A static_string_view object pointing to that storage.
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  * The view only remains valid for as long as the storage is kept alive.
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  *
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  * Note: using a std::string for the string-storage is not correct when that
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  * std::string object can be moved. This implementation is correct even when
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  * the storage-handler object gets moved.
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  */
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inline auto make_string_storage(std::string_view sv)
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{
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	auto storage = allocate_string_storage(sv.size());
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	char* p = storage.get();
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	std::ranges::copy(sv, p);
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	return std::pair{std::move(storage),
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	                 static_string_view(static_string_view::lifetime_ok_tag{},
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			                    std::string_view(p, sv.size()))};
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}
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#endif