sfinae hello world

README.adoc

@@ -20224,10 +20224,6 @@ Programs under link:userland/cpp/[] are examples of https://en.wikipedia.org/wik
 **** http://stackoverflow.com/questions/180172/default-constructor-with-empty-brackets
 ** `virtual` and polymorphism
 *** link:userland/cpp/virtual.cpp[]
-* templates
-** link:userland/cpp/template.cpp[]: basic example
-** link:userland/cpp/template_class_with_static_member.cpp[]: https://stackoverflow.com/questions/3229883/static-member-initialization-in-a-class-template
-** link:userland/cpp/if_constexpr.cpp[]: C++17 `if constexpr`: https://stackoverflow.com/questions/12160765/if-else-at-compile-time-in-c/54647315#54647315
 * iostream
 ** link:userland/cpp/copyfmt.cpp[]: `std::copyfmt` restores stream state, see also: https://stackoverflow.com/questions/12560291/set-back-default-floating-point-print-precision-in-c/53673686#53673686
 * fstream
@@ -20511,6 +20507,18 @@ https://stackoverflow.com/questions/81656/where-do-i-find-the-current-c-or-c-sta
 
 http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/n4659.pdf
 
+[[cpp-templates]]
+==== C++ templates
+
+* link:userland/cpp/template.cpp[]: basic example
+* link:userland/cpp/template_class_with_static_member.cpp[]: https://stackoverflow.com/questions/3229883/static-member-initialization-in-a-class-template
+
+===== SFINAE
+
+https://en.cppreference.com/w/cpp/language/sfinae
+
+Not possible to do the typecheck automatically without explicitly giving type constraints: https://stackoverflow.com/questions/53441832/sfinae-automatically-check-that-function-body-compiles-without-explicit-constrai
+
 [[cpp-type-casting]]
 ==== C++ type casting
 
@@ -20521,6 +20529,8 @@ https://stackoverflow.com/questions/332030/when-should-static-cast-dynamic-cast-
 [[cpp-compile-time-magic]]
 ==== C++ compile time magic
 
+* link:userland/cpp/if_constexpr.cpp[]: C++17 `if constexpr`: https://stackoverflow.com/questions/12160765/if-else-at-compile-time-in-c/54647315#54647315
+
 [[cpp-decltype]]
 ===== C++ `decltype`
 

userland/cpp/if_constexpr.cpp

@@ -1,5 +1,4 @@
-// https://cirosantilli.com/linux-kernel-module-cheat#cpp
+// https://cirosantilli.com/linux-kernel-module-cheat#cpp-compile-time-magic
-
 
 #if __cplusplus >= 201703L
 #include <cassert>

userland/cpp/sfinae.cpp

@@ -0,0 +1,40 @@
+// https://cirosantilli.com/linux-kernel-module-cheat#cpp-templates
+
+#include <cassert>
+
+struct HasMyfunc {
+    int myfunc() const { return 1; }
+};
+struct NoHasMyfunc {
+    int myfunc2() const { return 11; }
+};
+
+// int() is the actual return type.
+// t.myfunc() checks that t has method myfunc().
+// The decltype does not have multiple arguments, just a single comma separated expression:
+// https://stackoverflow.com/questions/16044514/what-is-decltype-with-two-arguments
+// Syntax breakdown:
+// https://medium.com/@mortificador/choose-between-different-implementations-depending-on-type-properties-at-compile-time-in-c-68e3fd5cd2f8
+template<class T>
+auto template_func(const T& t) ->
+    decltype(t.myfunc(), int())
+{
+    return t.myfunc();
+}
+
+template<class T>
+auto template_func(const T& t) ->
+    decltype(t.myfunc2(), int())
+{
+    return t.myfunc2();
+}
+
+int main() {
+    // Hello world example.
+    // The correct template is used for the type that has each different method.
+    // If we didn't specify constraints for each template type, C++ would blow up
+    // with multiple possible functions:
+    // https://stackoverflow.com/questions/53441832/sfinae-for-function-body
+    assert(template_func(HasMyfunc()) == 1);
+    assert(template_func(NoHasMyfunc()) == 11);
+}

userland/cpp/template.cpp

@@ -1,4 +1,4 @@
-// https://cirosantilli.com/linux-kernel-module-cheat#cpp
+// https://cirosantilli.com/linux-kernel-module-cheat#cpp-templates
 
 #include <cassert>
 

userland/cpp/template_class_with_static_member.cpp

@@ -1,4 +1,4 @@
-// https://cirosantilli.com/linux-kernel-module-cheat#cpp
+// https://cirosantilli.com/linux-kernel-module-cheat#cpp-templates
 
 #include <cassert>