/6/a396eece687d6cc18563188e015082da/p6_1.jpg) ## SFINAE ■ With templates, we have a technique called substitution failure is not an error (SFINAE). When the compiler looks into an instantiation of specialization for double. But what is with float? We can add another specialization, or we can use SFINAE. 2 bool equal(const T& a, const T& b) 4 return a == b; 7 template 8 bool equal(const } ## SFINAE ■ With templates, we have a technique called SFINAE. When the compiler looks into an instantiation of a template, and it turns

(Un-)Qualified Name Lookup Argument Dependent Lookup Two-Phase Lookup Template Argument Deduction SFINAE Overload Resolution (Viable) Candidate Functions Ranking Ambiguous Function Calls Access Labels (Un-)Qualified Name Lookup Argument Dependent Lookup Two-Phase Lookup Template Argument Deduction SFINAE Overload Resolution (Viable) Candidate Functions Ranking Ambiguous Function Calls Access Labels arguments. and add them to the overload set. Substitution errors are not necessarily flagged as error (SFINAE). ## Overview  ##

% clang++ -std=c++20 tests/c++/sfinae-ambiguous.cpp tests/c++/sfinae-ambiguous.cpp:10:5: error: no matching function for call to 'g' g(nullptr); tests/c++/sfinae-ambiguous.cpp:5:6: note: candidate % clang++ -std=c++20 tests/c++/sfinae-ambiguous.cpp tests/c++/sfinae-ambiguous.cpp:10:5: error: no matching function for call to 'g' g(nullptr); tests/c++/sfinae-ambiguous.cpp:5:6: note: candidate ambiguous auto g(T t) -> decltype(f(t)) { 1 error generated. % `cwhy --wrapper` -std=c++20 tests/c++/sfinae-ambiguous.cpp void f(void*) {} void f(char*) {} template auto g(T t)

## SFINAE template std::true_type can_have_pointer_to_member(int T::*); template std::false_type can_have_pointer_to_member(...); ## SFINAE template static_assert(decltype(can_have_pointer_to_member(nullptr))); ## SFINAE template std::true_type can_have_pointer_to_member(int T::*); template SFINAE template std::true_type can_have_pointer_to_member(int T::*); template std::false_type can_have_pointer_to_member(...); ## SFINAE template

Explicit Access - Easy Transition to Individual Objects • Less Powerful than the Singleton? ## SFINAE & Concepts ## Use Case • Function Requirements • Breaking SOONER in compile time ## Usage Example > ## Timeline SFINAE • ~2002 Concepts • C++20 ## Type Traits Introduced C++11 ## Original Trend – Substitution Failure is Not an Error (SFINAE) • Substitution Failure Is Not An Constraints on templates • Known for difficult to read errors • Difficult to constrain ## Original Code – SFINAE template

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interface of a class/function template ## 2 Embedded in a template signature ## 3 Simplify and extend SFINAE • disabling specific overloads for specific constraints (compared to std::enable_if) • constraints member types/functions existence (compared to void_t) • no dummy template parameters allocated for SFINAE needs • constraining function return types and deduced types of user's variables ## Benefits [Image](/uploads/documents/5/9/2/2/59224d14657be1430a61006e3f83332f/p124_3.jpg) ## 3 Simplify and extend SFINAE • disabling specific overloads for specific constraints (compared to std::enable_if) • constraints

ordering: https://godbolt.org/z/qvMcYn1Kd (if in both the namespace and the argument’s namespace) @@@ SFINAE is introduced by David Vandevoorde, to describe entity’s behavior If unqualified look finds an object overflow: what does it mean when something is SFINAE-friendly? (https://stackoverflow.com/questions/35033306/what-does-it-mean- when-one-says-something-is-sfinae-friendly) • Specializations and namespaces:

Adding a contract assertion can't affect Concepts / overload resolution / noexcept operator / SFINAE / if constexpr ... • Adding a contract assertion that is not checked can't cause runtime overhead Adding a contract assertion can't affect Concepts / overload resolution / noexcept operator / SFINAE / if constexpr ... • Adding a contract assertion that is not checked can't cause runtime overhead Adding a contract assertion can't affect Concepts / overload resolution / noexcept operator / SFINAE / if constexpr ... • Adding a contract assertion that is not checked can't cause runtime overhead

swap_pointed_toIdEvPT_S1_(double* a, double* b) { double temp = *a; *a = *b; *b = temp; } ## SFINAE Substitution Failure Is Not An Error A failure during substitution does not fail compilation Instead feature is necessary for function templates and class template partial specializations to be useful ## SFINAE Substitution Failure Is Not An Error A failure during substitution does not fail compilation Instead Substitution checks the declaration and template arguments Instantiation checks the entire definition SFINAE: Substitution Failure Is Not An Error Let the compiler deduce arguments for a function template

user defined conversions (25 pages) - template argument deduction (80 pages) - SFINAE (35 pages) - special member functions (30 pages) ☐ C++20 defines overload resolution in constructor explicit to prevent an implicit conversion ■ template functions can be eliminated through SFINAE - template functions which can not be instantiated will not be placed in the candidate set ■ convert