Brown Bag #2 Advanced C++

Topics  Templates  Standard Template Library (STL)  Pointers and Smart Pointers  Exceptions  Lambda Expressions  Tips and Tricks!

Templates  Generic code that works with many data types  Encourages code reuse  Turing-complete  Template metaprogramming (not covered)  Beware scary compiler/linker errors

Function Templates  A family of functions  Uses a generic type  On demand compilation  Compiler can deduce types  Type-safe int Square(int num) { return num * num; } template T Square(T num) { return num * num; } float result = Square(5.0f); int result = Square(2);

Class Templates  Explicit type specification  Declaration + implementation in same file  Can template methods not just whole classes  Great for containers template class Thing { public: Thing(T data) : m_data(data) {} T GetData() const { return m_data; } private: T m_data; }; Thing myThing = Thing (50); int data = myThing.GetData();  class and typename are interchangeable

Standard Template Library (STL)  Containers  vector  list  map  string  Algorithms  for_each  find  sort  Iterators  auto keyword (not directly relevant, but handy)

vector  Dynamic array  Access item at index = constant time  Iterate over all elements = linear time for ( auto iter = myVector.begin(); iter != myVector.end(); ++iter ) { iter->foo(); } std::vector myVector; std::vector ::iterator myVector[0].foo();

for_each #include using namespace std; void myFunction (int i) { cout << " " << i; } int main() { vector myVector; myVector.push_back(10); myVector.push_back(20); for_each (myVector.begin(), myVector.end(), myFunction); }

string  Special container type – sequence of characters.  Contains useful functions and common STL container functionality. #include int main() { std::string test(“Hello World!”); std::cout << “Length of string is “ << test.size() << “.\n”; // 12 }

map  Associative container that stores values as a pair.  An array uses an integer as the key type.  Each element must have a unique key.  Map containers support iterators that return key and value. #include using namespace std; int main() { map testMap; testMap.insert(map ::value_type(“Hello”, 5); int myInt = testMap[“Hello”]; cout << “Value contained in element with key ‘Hello’ is “ << myInt << “.\n”; // 5 }

Pointers  Pointers are references to memory blocks which contain data (or an instruction).  We access this data using the reference (&) and dereference (*) operators.

Dereference Operator (*)  If a pointer is a memory address, how do we access the object at that location?  Dereference a pointer to obtain the value at the memory address.

Reference Operator (&)  How do we alter a value at a given memory address and not just a copy?  Use the Reference operator to obtain a variable's memory address.

Pointer to a Pointer  It is possible to have a pointer that points to another pointer.

Pointer to a Pointer  Ever seen a DirectX function where you pass in a reference to a pointer? Check the argument list - that's what is happening there!

Class and Struct Pointers  You can create pointers to struct and class objects using the 'new' keyword:  Lets try setting the value of ack::bar to 5:

Class and Struct Pointers  This is C# syntax - doesn't work in C++!

The -> Operator  Like before, we must dereference the pointer before we can access the object!  There's a nicer way:  The -> operator dereferences a class or struct pointer and gives access to its members.  This is known as "syntactic sugar".

‘delete’ and Null Pointers  When you de-allocate a pointer using the 'delete' keyword, it is common to set the pointer's value to 0:  A pointer whose value is 0 is known as a null pointer.

Smart Pointers  These can be found in the Standard Library as part of the header file.  There are three types:  unique_ptr  shared_ptr  weak_ptr

Smart Pointer Syntax  Pointers declared using template-style syntax:  * and & operators can be utilised as normal:  Memory is de-allocated at the end:  However, de-allocation does not need to be done manually!

Reference Counting  Smart pointers count the number of references to an object in memory.  When a pointer leaves scope, the reference count is decremented.  When the reference count reaches 0, the memory is de-allocated.

unique_ptr  Allows for only one reference to a stored object - it is unique.  This is an invalid operation. However, ownership can be transferred:  When memory is de-allocated:

shared_ptr  shared_ptrs allow for multiple pointers to reference the same memory address.

weak_ptr  To avoid circular references, we use weak_ptrs:  In order to access the shared_ptr, we use weak_ptr::lock():  When the shared_ptr is deallocated, weak_ptr::lock() will return an empty shared_ptr object:

nullptr  Traditionally, a null pointer is defined as NULL, or 0. Consider the following:  How do we distinguish between 0 and a null pointer?  C++11 introduces the nullptr type:  Now we no longer need to worry about confusing null pointers and int values!

Smart Pointers: Summary  Smart pointers allow for all the same functionality of a standard pointer.  Makes use of Reference Counting for automatic de-allocation.  unique_ptr makes it easier to store single references to objects at a time.  shared_ptr allows for multiple pointers to share memory.  weak_ptr allows for accessing shared_ptr objects with easy clean-up.  nullptr helps to clearly distinguish from numeric values.

Simples!

Exceptions  Handle exceptional runtime errors  Unwinds stack (releases local variables, etc.)  Used by standard library / STL  Standard exceptions (bad_alloc, bad_cast, etc.)  Custom exceptions (extend std::exception )  Exception specifications try { throw 20; } catch (int e) { cout << “Exception:" << e << endl; } float foo(char param) throw (int);

Exceptions: The Good  Cleaner than error codes  Much nicer for deeply-nested functions  Separates error-handling from program flow  User-definable to carry detailed information  Catch constructor errors

Resource Acquisition Is Initialization (RAII)  Only destructors are guaranteed to run after an exception is hit  Use destructors to prevent resources leaks  Doesn’t require messy try/catch blocks void foo(void) { std::unique_ptr myThing( new Thing() ); myThing->Something(); }

Exceptions: The Bad, The Ugly  Multiple program exit points  Changes program flow, maybe harder to debug  Make debugger break on exceptions in Debug  Potential to leak resources if misused  Use smart pointers, etc. to avoid  Exception-safe code can be hard to write  Don’t throw in destructors  Only throw on exceptional errors  Hard to introduce to existing code

Lambda Expressions [ ] () mutable throw() –> int { }  Related to the concept of anonymous functions.  Helps to solve the problems of function objects and function pointers.  Function pointer has minimal syntactic overhead but does not retain state.  Function object retains state but requires the overhead of a class definition.  Lambdas feature minimal overhead and can retain state within the scope in which they are defined.

Lambda Expressions: Example void LambdaExample() { auto myLambda = [](int x, int y) -> int { return (x * 2) + y; }; int a = 3; int b = 4; int c = myLambda(a, b); std::cout << “The value of c is “ << c << “.\n”; // 10 int d = myLambda(c, b); std::cout << “The value of d is “ << d << “.\n”; // 24 }

Lambda Expressions: Syntax  Capture Clause: [ ]  Used to access variables from the scope enclosing the lambda.  Can be passed by reference or value (e.g. &x, y).  Default capture mode can be specified using & or = at the beginning for reference or value captures respectively (e.g. [&, x] or [=, y]).  ‘this’ pointer provides access to member variables of the enclosing class (e.g. [this]).  Parameter List: ()  Specifies the parameters passed into the function, as with a regular function declaration.  Mutable Specification: mutable  Allows values captured by reference to be modified within the function.  Will not change the original value, only the local copy.

Lambda Expressions: Syntax (cont.)  Throw Specification: throw()  Specifies if the lambda can throw an exception.  throw() specifies no exception can be thrown.  throw(T) specifies an exception of type T can be thrown.  Return Type: -> T  Follows trailing return-type syntax introduced in C++11.  Explicitly specifies the return value of the function.  Can be implicitly implied via a return statement in the function body.  Function Body: { }  Defines the instructions to be performed, as with a standard function.

Lambda Expressions: Example Revisited void LambdaExample() { auto myLambda = [](int x, int y) -> int { return (x * 2) + y; }; int a = 3; int b = 4; int c = myLambda(a, b); std::cout << “The value of c is “ << c << “.\n”; // 10 int d = myLambda(c, b); std::cout << “The value of d is “ << d << “.\n”; // 24 }

Why use Lambda Expressions?  Iterator functions: #include int main() { std::vector myVector; myVector.push_back(10); myVector.push_back(20); int totalCount = 0; for_each (myVector.begin(), myVector.end(), [&totalCount](int x) { totalCount += x; }); std::cout << “The total of all values in myVector is “ << totalCount << “.\n”; // 30 }

Why use Lambda Expressions?  Asynchronous tasks: #include using namespace Concurrency; int main() { auto doubleNum = [](int x) { return x * 2; }; auto incrementNum = [](int x) { return ++x; }; auto startTask = create_task([]() -> int { return 5; }); int finalNum = startTask.then(doubleNum).then(incrementNum).then(doubleNum).get(); std::cout << “The value of finalNum is “ << finalNum << “.\n”; // 22 }

Tips and Tricks

Const FTW  Prefer pass-by-reference-to-const to pass-by-value (item #20)  Avoid unnecessary constructors/destructor calls  Still guarantee to caller that object won’t be changed void Foo( const Thing& input ); Thing GetData() const;  const member functions (getters)

Enums FTW struct MyEnum { enum Enum { MAX }; enum class MyEnum { MAX };  Nicer and safer than pre-processor definitions  Enum classes/structs (C++ 11)  Old: Wrap Enums in struct  Now type-safe in C++ 11

Further Reading  Microsoft Developers Network (MSDN)  CPlusPlus.com