cpp: initializer list constructor

userland/cpp/initializer_list_constructor.cpp

@@ -0,0 +1,105 @@
+// https://cirosantilli.com/linux-kernel-module-cheat#cpp
+//
+// # Brace enclosed initializer list
+//
+// # List initialization
+//
+// # Initializer list constructor
+//
+// Applications:
+//
+// -   you don't know beforehand how many arguments a constructor should receive
+//
+//     For example, the stdlib std::vector class gets an initializer list constructor on C++11,
+//     which allows one to initialize it to any constant.
+//
+//     TODO could this not be achieved via cstdarg?
+
+#include <cassert>
+#include <initializer_list>
+#include <vector>
+
+int main() {
+#if __cplusplus >= 201103L
+    // STL std::vector usage example
+    {
+        std::vector<int> v{0, 1};
+        // SAME.
+        //std::vector<int> v = std::vector<int>({0, 1});
+        assert(v[0] == 0);
+        assert(v[1] == 1);
+        assert(v == std::vector<int>({0, 1}));
+        assert((v == std::vector<int>{0, 1}));
+
+        // Assignment also works via implicit conversion.
+        v = {1, 0};
+        assert((v == std::vector<int>{1, 0}));
+
+        // ERROR: TODO why no implicit conversion is made?
+        //assert((v == {0, 1}));
+    }
+
+    // How to implement one yourself.
+    {
+        struct InitializerListCtor {
+            std::vector<int> v;
+
+            InitializerListCtor(int i, int j) {
+                v.push_back(i);
+                v.push_back(j + 1);
+            }
+
+            InitializerListCtor(std::initializer_list<int> list) {
+                for (auto& i : list)
+                    v.push_back(i);
+            }
+
+            InitializerListCtor(int before, std::initializer_list<int> list, int after) {
+                v.push_back(before + 1);
+                for (auto& i : list)
+                    v.push_back(i);
+                v.push_back(after - 1);
+            }
+        };
+
+        // Initializer list constructor is called, not the (int,int) one.
+        {
+            InitializerListCtor o{0, 1};
+            assert((o.v == std::vector<int>{0, 1}));
+        }
+
+        // 3 param constructor is called
+        {
+            InitializerListCtor o(0, {0, 0,}, 0);
+            assert((o.v == std::vector<int>{1, 0, 0, -1}));
+        }
+    }
+
+    // # auto and initializer lists
+    //
+    // auto rule: brace initializer can be bound to auto
+    //
+    // http://en.cppreference.com/w/cpp/utility/initializer_list
+    //
+    {
+        {
+            // TODO GCC 5.1 does not allow this, which conflicts with
+            // http://en.cppreference.com/w/cpp/utility/initializer_list
+            // Who is right?
+            // auto InitializerListCtor{0, 1, 2};
+            // SAME:
+            //initializer_list<int> l{0, 1, 2};
+            //assert(l.size() == 3);
+            //assert(*l.begin() == 0);
+        }
+
+        // The rule for auto makes this ranged for work.
+        // TODO why here? I see an `int`, not an `auto`
+        int i = 0;
+        for (auto x : {0, 1, 2}) {
+            assert(x == i);
+            i++;
+        }
+    }
+#endif
+}

userland/cpp/most_vexing_parse.cpp

@@ -1,17 +1,20 @@
 // https://cirosantilli.com/linux-kernel-module-cheat#cpp
 
-#include "common.hpp"
+#include <cassert>
 
 int main() {
 
+    struct D {
+        int i;
+        D() {}
+        D(int i) : i(i) {}
+    };
+
     struct C {
         int i;
         C() : i(1) {}
         C(int i) : i(i) {}
-    };
-
-    struct D {
-        D() {}
+        C(const D& d) : i(d.i) {}
     };
 
     // Declares *FUNCTION* called `c` that returns `C` inside function main.
@@ -46,12 +49,36 @@ int main() {
     }
 
     // But sometimes even arguments are not enough: here D()
-    // could matn that the declared `c`
+    // is interpreted as "a function of type `D f()`"
     {
-        C c(D());
+        C c(D(2));
 #if 0
         // error: request for member ‘i’ in ‘c’, which is of non-class type ‘main()::C(main()::D (*)())’
         assert(c.i == 2);
 #endif
     }
+
+    // Solving the most vexing parse.
+    // https://stackoverflow.com/questions/13249694/avoid-the-most-vexing-parse
+    {
+        // Extra parenthesis.
+        {
+            C c((D(2)));
+            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);
+        }
+
+        // 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);
+        }
+    }
 }