learn more c++, it never ends

userland/cpp/most_vexing_parse.cpp

@@ -1,20 +1,18 @@
 // https://cirosantilli.com/linux-kernel-module-cheat#cpp
 
-#include <cassert>
-
 int main() {
 
     struct D {
         int i;
-        D() {}
-        D(int i) : i(i) {}
+        constexpr D() : i(0) {}
+        constexpr D(int i) : i(i) {}
     };
 
     struct C {
         int i;
-        C() : i(1) {}
-        C(int i) : i(i) {}
-        C(const D& d) : i(d.i) {}
+        constexpr C() : i(1) {}
+        constexpr C(int i) : i(i) {}
+        constexpr C(const D& d) : i(d.i) {}
     };
 
     // Declares *FUNCTION* called `c` that returns `C` inside function main.
@@ -25,7 +23,7 @@ int main() {
     // Therefore there would be not way for C++ to distinguish between the two,
     // and still be backwards compatible with C.
     {
-        C c();
+        constexpr C c();
 
 #if 0
         // ERROR: function definition is not possible inside another function.
@@ -37,24 +35,24 @@ int main() {
 
     // If you want to call a default constructor, use:
     {
-        C c;
-        assert(c.i == 1);
+        constexpr C c;
+        static_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);
+        constexpr C c(2);
+        static_assert(c.i == 2);
     }
 
     // But sometimes even arguments are not enough: here D()
     // is interpreted as "a function of type `D f()`"
     {
-        C c(D(2));
+        constexpr 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);
+        static_assert(c.i == 0);
 #endif
     }
 
@@ -63,22 +61,22 @@ int main() {
     {
         // Extra parenthesis.
         {
-            C c((D(2)));
-            assert(c.i == 2);
+            constexpr C c((D(2)));
+            static_assert(c.i == 2);
         }
 
         // Initialize through assignment. TODO likely guaranteed to be cost-free,
         // but confirm.
         {
-            C c = C((D(2)));
-            assert(c.i == 2);
+            constexpr C c = C((D(2)));
+            static_assert(c.i == 2);
         }
 
         // Initializer list. Only works if there is no initializer_list constructor.
         // Only works in general if c does not have an ambiguous initializer_list constructor though.
         {
-            C c{D(2)};
-            assert(c.i == 2);
+            constexpr C c{D(2)};
+            static_assert(c.i == 2);
         }
     }
 }