Ensure no copy is made when using argsToString()

It seems that std::make_tuple() is using __decay_and_strip so
the arguments get copied. Using the std::tuple c'tor directly
instead.

When using the %-operator it is already taken care that strings
are stored as pointers and not by value.
charconv
Martchus 3 years ago
parent c7c5352325
commit 8744cf95ef
  1. 7
      conversion/stringbuilder.h
  2. 26
      tests/conversiontests.cpp

@ -241,6 +241,11 @@ constexpr void append(StringType &target, TupleType &&tuple, typename StringType
return TupleToString<StringType, TupleType, std::tuple_size_v<std::decay_t<TupleType>>>::append(std::forward<TupleType>(tuple), target);
}
template <typename... Elements> constexpr std::tuple<Elements &&...> makeTupleHoldingRefsToTemporaryObjects(Elements &&... args) noexcept
{
return std::tuple<Elements &&...>(std::forward<Elements>(args)...);
}
} // namespace Helper
/// \endcond
@ -257,7 +262,7 @@ template <class StringType = std::string, class... Args> inline StringType tuple
template <class StringType = std::string, class... Args> inline StringType argsToString(Args &&... args)
{
return tupleToString(std::make_tuple(std::forward<Args>(args)...));
return tupleToString(Helper::makeTupleHoldingRefsToTemporaryObjects(std::forward<Args>(args)...));
}
/*!

@ -387,6 +387,23 @@ struct ConvertibleToString {
operator std::string() const;
};
struct StringThatDoesNotLikeToBeCopiedOrMoved : public std::string {
explicit StringThatDoesNotLikeToBeCopiedOrMoved(const char *value)
: std::string(value)
{
}
[[noreturn]] StringThatDoesNotLikeToBeCopiedOrMoved(const StringThatDoesNotLikeToBeCopiedOrMoved &other)
: std::string(other)
{
CPPUNIT_FAIL("attempt to copy string: " + other);
}
[[noreturn]] StringThatDoesNotLikeToBeCopiedOrMoved(StringThatDoesNotLikeToBeCopiedOrMoved &&other)
: std::string(std::move(other))
{
CPPUNIT_FAIL("attempt to move string: " + other);
}
};
void ConversionTests::testStringBuilder()
{
// check whether type traits work as expected
@ -398,7 +415,8 @@ void ConversionTests::testStringBuilder()
static_assert(Helper::IsStringViewType<std::wstring, std::wstring_view>::value);
static_assert(Helper::IsConvertibleToConstStringRef<std::string, ConvertibleToString>::value);
#ifdef CPP_UTILITIES_USE_STANDARD_FILESYSTEM
static_assert(!Helper::IsConvertibleToConstStringRef<std::filesystem::path::string_type, std::filesystem::path>::value, "conversion via native() preferred");
static_assert(!Helper::IsConvertibleToConstStringRef<std::filesystem::path::string_type, std::filesystem::path>::value,
"conversion via native() preferred");
#endif
static_assert(
!Helper::IsConvertibleToConstStringRef<std::string, std::string>::value, "yes, in this context this should not be considered convertible");
@ -428,4 +446,10 @@ void ConversionTests::testStringBuilder()
CPPUNIT_ASSERT_EQUAL_MESSAGE(
"regular + operator still works (no problems with ambiguity)"s, "regular + still works"s, "regular"s + " + still works");
CPPUNIT_ASSERT_EQUAL_MESSAGE("using string_view", "foobar123"s, "foo"sv % "bar"sv + 123);
// check that for the internal tuple construction no copies are made
StringThatDoesNotLikeToBeCopiedOrMoved str(" happen ");
const StringThatDoesNotLikeToBeCopiedOrMoved str2("for this");
CPPUNIT_ASSERT_EQUAL("no copy/move should happen for this!"s,
argsToString(StringThatDoesNotLikeToBeCopiedOrMoved("no copy/move should"), str, str2, StringThatDoesNotLikeToBeCopiedOrMoved("!")));
}

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