Merge branch 'master' of github.com:adambard/learnxinyminutes-docs

1 files changed, 77 insertions, 5 deletions

diff --git a/c++.html.markdown b/c++.html.markdown
index ff2a98fd..8a7f5a59 100644
--- a/c++.html.markdown
+++ b/c++.html.markdown
@@ -4,6 +4,7 @@ filename: learncpp.cpp
 contributors:
     - ["Steven Basart", "http://github.com/xksteven"]
     - ["Matt Kline", "https://github.com/mrkline"]
+    - ["Geoff Liu", "http://geoffliu.me"]
 lang: en
 ---
 
@@ -158,11 +159,12 @@ void foo()
 
 int main()
 {
-    // Assume everything is from the namespace "Second"
-    // unless otherwise specified.
+    // Includes all symbols from `namesapce Second` into the current scope. Note
+    // that simply `foo()` no longer works, since it is now ambiguous whether
+    // we're calling the `foo` in `namespace Second` or the top level.
     using namespace Second;
 
-    foo(); // prints "This is Second::foo"
+    Second::foo(); // prints "This is Second::foo"
     First::Nested::foo(); // prints "This is First::Nested::foo"
     ::foo(); // prints "This is global foo"
 }
@@ -248,6 +250,59 @@ const string& barRef = bar; // Create a const reference to bar.
 // Like C, const values (and pointers and references) cannot be modified.
 barRef += ". Hi!"; // Error, const references cannot be modified.
 
+// Sidetrack: Before we talk more about references, we must introduce a concept
+// called a temporary object. Suppose we have the following code:
+string tempObjectFun() { ... }
+string retVal = tempObjectFun();
+
+// What happens in the second line is actually:
+//   - a string object is returned from `tempObjectFun`
+//   - a new string is constructed with the returned object as arugment to the
+//     constructor
+//   - the returned object is destroyed
+// The returned object is called a temporary object. Temporary objects are
+// created whenever a function returns an object, and they are destroyed at the
+// end of the evaluation of the enclosing expression (Well, this is what the
+// standard says, but compilers are allowed to change this behavior. Look up
+// "return value optimization" if you're into this kind of details). So in this
+// code:
+foo(bar(tempObjectFun()))
+
+// assuming `foo` and `bar` exist, the object returned from `tempObjectFun` is
+// passed to `bar`, and it is destroyed before `foo` is called.
+
+// Now back to references. The exception to the "at the end of the enclosing
+// expression" rule is if a temporary object is bound to a const reference, in
+// which case its life gets extended to the current scope:
+
+void constReferenceTempObjectFun() {
+  // `constRef` gets the temporary object, and it is valid until the end of this
+  // function.
+  const string& constRef = tempObjectFun();
+  ...
+}
+
+// Another kind of reference introduced in C++11 is specifically for temporary
+// objects. You cannot have a variable of its type, but it takes precedence in
+// overload resolution:
+
+void someFun(string& s) { ... }  // Regular reference
+void someFun(string&& s) { ... }  // Reference to temporary object
+
+string foo;
+someFun(foo);  // Calls the version with regular reference
+someFun(tempObjectFun());  // Calls the version with temporary reference
+
+// For example, you will see these two versions of constructors for
+// std::basic_string:
+basic_string(const basic_string& other);
+basic_string(basic_string&& other);
+
+// Idea being if we are constructing a new string from a temporary object (which
+// is going to be destroyed soon anyway), we can have a more efficient
+// constructor that "salvages" parts of that temporary string. You will see this
+// concept referred to as the move semantic.
+
 //////////////////////////////////////////
 // Classes and object-oriented programming
 //////////////////////////////////////////
@@ -680,7 +735,7 @@ class Foo {
   virtual void bar();
 };
 class FooSub : public Foo {
-  virtual void bar();  // overrides Foo::bar!
+  virtual void bar();  // Overrides Foo::bar!
 };
 
 
@@ -699,7 +754,8 @@ pt2 = nullptr;  // Sets pt2 to null.
 
 
 // '=' != '=' != '='!
-// Calls Foo::Foo(const Foo&) or some variant copy constructor.
+// Calls Foo::Foo(const Foo&) or some variant (see move semantics) copy
+// constructor.
 Foo f2;
 Foo f1 = f2;
 
@@ -713,6 +769,22 @@ Foo f1 = fooSub;
 Foo f1;
 f1 = f2;
 
+
+// How to truly clear a container:
+class Foo { ... };
+vector<Foo> v;
+for (int i = 0; i < 10; ++i)
+  v.push_back(Foo());
+
+// Following line sets size of v to 0, but destructors don't get called,
+// and resources aren't released!
+v.empty();
+v.push_back(Foo());  // New value is copied into the first Foo we inserted in the loop.
+
+// Truly destroys all values in v. See section about temporary object for
+// explanation of why this works.
+v.swap(vector<Foo>());
+
 ```
 Further Reading: