diff options
Diffstat (limited to 'c++.html.markdown')
| -rw-r--r-- | c++.html.markdown | 138 |
1 files changed, 110 insertions, 28 deletions
diff --git a/c++.html.markdown b/c++.html.markdown index ff2a98fd..4acc1b9d 100644 --- a/c++.html.markdown +++ b/c++.html.markdown @@ -4,6 +4,8 @@ filename: learncpp.cpp contributors: - ["Steven Basart", "http://github.com/xksteven"] - ["Matt Kline", "https://github.com/mrkline"] + - ["Geoff Liu", "http://geoffliu.me"] + - ["Connor Waters", "http://github.com/connorwaters"] lang: en --- @@ -52,11 +54,11 @@ int main(int argc, char** argv) // However, C++ varies in some of the following ways: -// In C++, character literals are one byte. -sizeof('c') == 1 +// In C++, character literals are chars +sizeof('c') == sizeof(char) == 1 -// In C, character literals are the same size as ints. -sizeof('c') == sizeof(10) +// In C, character literals are ints +sizeof('c') == sizeof(int) // C++ has strict prototyping @@ -158,11 +160,12 @@ void foo() int main() { - // Assume everything is from the namespace "Second" - // unless otherwise specified. + // Includes all symbols from namespace 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 +251,59 @@ 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 "move semantics". + ////////////////////////////////////////// // Classes and object-oriented programming ////////////////////////////////////////// @@ -294,7 +350,10 @@ public: // These are called when an object is deleted or falls out of scope. // This enables powerful paradigms such as RAII // (see below) - // Destructors must be virtual to allow classes to be derived from this one. + // The destructor should be virtual if a class is to be derived from; + // if it is not virtual, then the derived class' destructor will + // not be called if the object is destroyed through a base-class reference + // or pointer. virtual ~Dog(); }; // A semicolon must follow the class definition. @@ -437,9 +496,10 @@ int main () { ///////////////////// // Templates in C++ are mostly used for generic programming, though they are -// much more powerful than generics constructs in other languages. It also -// supports explicit and partial specialization, functional-style type classes, -// and also it's Turing-complete. +// much more powerful than generic constructs in other languages. They also +// support explicit and partial specialization and functional-style type +// classes; in fact, they are a Turing-complete functional language embedded +// in C++! // We start with the kind of generic programming you might be familiar with. To // define a class or function that takes a type parameter: @@ -451,7 +511,7 @@ public: }; // During compilation, the compiler actually generates copies of each template -// with parameters substituted, and so the full definition of the class must be +// with parameters substituted, so the full definition of the class must be // present at each invocation. This is why you will see template classes defined // entirely in header files. @@ -465,13 +525,13 @@ intBox.insert(123); Box<Box<int> > boxOfBox; boxOfBox.insert(intBox); -// Up until C++11, you must place a space between the two '>'s, otherwise '>>' -// will be parsed as the right shift operator. +// Until C++11, you had to place a space between the two '>'s, otherwise '>>' +// would be parsed as the right shift operator. // You will sometimes see // template<typename T> -// instead. The 'class' keyword and 'typename' keyword are _mostly_ -// interchangeable in this case. For full explanation, see +// instead. The 'class' keyword and 'typename' keywords are _mostly_ +// interchangeable in this case. For the full explanation, see // http://en.wikipedia.org/wiki/Typename // (yes, that keyword has its own Wikipedia page). @@ -527,12 +587,15 @@ try { // Do not allocate exceptions on the heap using _new_. throw std::runtime_error("A problem occurred"); } + // Catch exceptions by const reference if they are objects catch (const std::exception& ex) { - std::cout << ex.what(); + std::cout << ex.what(); +} + // Catches any exception not caught by previous _catch_ blocks -} catch (...) +catch (...) { std::cout << "Unknown exception caught"; throw; // Re-throws the exception @@ -542,8 +605,8 @@ catch (const std::exception& ex) // RAII /////// -// RAII stands for Resource Allocation Is Initialization. -// It is often considered the most powerful paradigm in C++, +// RAII stands for "Resource Acquisition Is Initialization". +// It is often considered the most powerful paradigm in C++ // and is the simple concept that a constructor for an object // acquires that object's resources and the destructor releases them. @@ -564,9 +627,9 @@ void doSomethingWithAFile(const char* filename) // Unfortunately, things are quickly complicated by error handling. // Suppose fopen can fail, and that doSomethingWithTheFile and // doSomethingElseWithIt return error codes if they fail. -// (Exceptions are the preferred way of handling failure, -// but some programmers, especially those with a C background, -// disagree on the utility of exceptions). +// (Exceptions are the preferred way of handling failure, +// but some programmers, especially those with a C background, +// disagree on the utility of exceptions). // We now have to check each call for failure and close the file handle // if a problem occurred. bool doSomethingWithAFile(const char* filename) @@ -680,26 +743,29 @@ class Foo { virtual void bar(); }; class FooSub : public Foo { - virtual void bar(); // overrides Foo::bar! + virtual void bar(); // Overrides Foo::bar! }; // 0 == false == NULL (most of the time)! bool* pt = new bool; -*pt = 0; // Sets the value points by 'pt' to false. +*pt = 0; // Sets the value points by 'pt' to false. pt = 0; // Sets 'pt' to the null pointer. Both lines compile without warnings. // nullptr is supposed to fix some of that issue: int* pt2 = new int; -*pt2 = nullptr; // Doesn't compile +*pt2 = nullptr; // Doesn't compile pt2 = nullptr; // Sets pt2 to null. -// But somehow 'bool' type is an exception (this is to make `if (ptr)` compile). +// There is an exception made for bools. +// This is to allow you to test for null pointers with if(!ptr), +// but as a consequence you can assign nullptr to a bool directly! *pt = nullptr; // This still compiles, even though '*pt' is a bool! // '=' != '=' != '='! -// 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 +779,22 @@ 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 + +// Truly destroys all values in v. See section about temporary objects for +// explanation of why this works. +v.swap(vector<Foo>()); + ``` Further Reading: |