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authorGeoff Liu <g@geoffliu.me>2015-08-30 14:20:18 -0600
committerGeoff Liu <g@geoffliu.me>2015-08-30 14:20:18 -0600
commit1d1def16a5d7925bb8f7fba7dc49182e33359e85 (patch)
treee67871bef7d825ac7f5257f9325c040cd1f83807 /c++.html.markdown
parenta6af51e42f6eef190fe8bdc602a7c07f040d4f5e (diff)
A little more about C++ references
Diffstat (limited to 'c++.html.markdown')
-rw-r--r--c++.html.markdown51
1 files changed, 51 insertions, 0 deletions
diff --git a/c++.html.markdown b/c++.html.markdown
index ff2a98fd..efce0053 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
---
@@ -248,6 +249,56 @@ 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. 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
//////////////////////////////////////////