cpp: most vexing parse

README.adoc

@@ -14475,6 +14475,10 @@ Programs under link:userland/cpp/[] are examples of https://en.wikipedia.org/wik
 ** associative
 *** <<algorithms>> contains a benchmark comparison of different c++ containers
 *** link:userland/cpp/set.cpp[]: `std::set` contains unique keys
+* Language madness
+** link:userland/cpp/most_vexing_parse.cpp[]: the most vexing parse is a famous constructor vs function declaration syntax gotcha!
+*** https://en.wikipedia.org/wiki/Most_vexing_parse
+*** http://stackoverflow.com/questions/180172/default-constructor-with-empty-brackets
 
 [[cpp-multithreading]]
 ==== C++ multithreading

userland/cpp/most_vexing_parse.cpp

@@ -0,0 +1,57 @@
+// https://cirosantilli.com/linux-kernel-module-cheat#cpp
+
+#include "common.hpp"
+
+int main() {
+
+    struct C {
+        int i;
+        C() : i(1) {}
+        C(int i) : i(i) {}
+    };
+
+    struct D {
+        D() {}
+    };
+
+    // Declares *FUNCTION* called `c` that returns `C` inside function main.
+    //
+    // This is the same as in C, where it is possible to declare a function from inside another function,
+    // but not define it.
+    //
+    // Therefore there would be not way for C++ to distinguish between the two,
+    // and still be backwards compatible with C.
+    {
+        C c();
+
+#if 0
+        // ERROR: function definition is not possible inside another function.
+        C c() {return C();}
+#endif
+
+        //c.i;
+    }
+
+    // If you want to call a default constructor, use:
+    {
+        C c;
+        assert(c.i == 1);
+    }
+
+    // For non-default constructors, literal arguments disambiguate
+    // things as this syntax could not possibly be a function declaration.
+    {
+        C c(2);
+        assert(c.i == 2);
+    }
+
+    // But sometimes even arguments are not enough: here D()
+    // could matn that the declared `c`
+    {
+        C c(D());
+#if 0
+        // error: request for member ‘i’ in ‘c’, which is of non-class type ‘main()::C(main()::D (*)())’
+        assert(c.i == 2);
+#endif
+    }
+}