decltype

README.adoc

@@ -20518,6 +20518,20 @@ link:userland/cpp/static_dynamic_reinterpret_cast.cpp[]
 
 https://stackoverflow.com/questions/332030/when-should-static-cast-dynamic-cast-const-cast-and-reinterpret-cast-be-used/60414256#60414256
 
+[[cpp-compile-time-magic]]
+==== C++ compile time magic
+
+[[cpp-decltype]]
+===== C++ `decltype`
+
+link:userland/cpp/decltype.cpp[]
+
+C++11 keyword.
+
+Replaces decltype with type of an expression at compile time.
+
+More powerful than `auto` as you can use it in more places.
+
 === POSIX
 
 Programs under link:userland/posix/[] are examples of POSIX C programming.

userland/cpp/decltype.cpp

@@ -0,0 +1,69 @@
+// https://cirosantilli.com/linux-kernel-module-cheat#cpp-decltype
+
+#include <cassert>
+#include <vector>
+#include <utility> // declval
+
+int f() {
+    return 1;
+}
+
+class C {
+    public:
+        int f() { return 2; }
+};
+
+int i;
+decltype(i) g() {
+    return 1;
+}
+
+int main() {
+#if __cplusplus >= 201103L
+    // Implies reference while auto does not.
+    {
+        int i = 0;
+        int& ir = i;
+        decltype(ir) ir2 = ir;
+        ir2 = 1;
+        assert(i == 1);
+    }
+
+    // Can be used basically anywhere.
+    {
+        int i = 0;
+        std::vector<decltype(i)> v;
+        v.push_back(0);
+    }
+
+    // Return value.
+    {
+        decltype(f()) i;
+        assert(typeid(i) == typeid(int));
+
+        C c;
+        decltype(c.f()) j;
+        assert(typeid(j) == typeid(int));
+
+        // Return value without instance. Use declval.
+        // http://stackoverflow.com/questions/9760358/decltype-requires-instantiated-object
+        decltype(std::declval<C>().f()) k;
+        assert(typeid(k) == typeid(int));
+    }
+
+    // decltype must take expressions as input, not a type.
+    // For types with the default constructor like `int`, we can just call the default constructor as in int():
+    // https://stackoverflow.com/questions/39279074/what-does-the-void-in-decltypevoid-mean-exactly
+    // or we can just use declval.
+    {
+        decltype(int()) i = 0;
+        assert(typeid(i) == typeid(int));
+
+        decltype(std::declval<int>()) j = 0;
+        assert(typeid(j) == typeid(int));
+    }
+
+    // Can be used to declare the return value of functions.
+    assert(g() == 1);
+#endif
+}