Add "explicit" keyword for several unary constructors.

We recommend to use explicit for unary constructors, but we are not
using is in the following slides. Here, it is added in several places
where the addition doesn't create space issues on the slide.

Author: hageboeck

talk/objectorientation/constructors.tex

@@ -226,7 +226,7 @@
     \begin{cppcode}
       struct Delegate {
         int m_i;
-        Delegate(int i) : m_i(i) {
+        explicit Delegate(int i) : m_i(i) {
           ... complex initialization ...
         }
         Delegate() : Delegate(42) {}
@@ -247,7 +247,7 @@
   \begin{exampleblock}{Practically}
     \begin{cppcode}
       struct Base {
-        Base(int a);           // ctor 1
+        explicit Base(int a);  // ctor 1
       };
       struct Derived : Base {
         using Base::Base;
@@ -270,9 +270,9 @@
   \begin{exampleblock}{Practically}
     \begin{cppcode}
       struct Base {
-        int a{5}; // also possible: int a = 5;
-        Base() = default;
-        Base(int _a) : a(_a) {}
+        int a{5}; // or: int a = 5;
+                 Base() = default;
+        explicit Base(int _a) : a(_a) {}
       };
       struct Derived : Base {
         int b{6};