Difference between revisions of "Intermediate C++ Game Programming Tutorial 20"
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** If a class only contain data members (Types) that handle their own RAII behavior (such as the STL containers), then you don't need to supply any move member functions | ** If a class only contain data members (Types) that handle their own RAII behavior (such as the STL containers), then you don't need to supply any move member functions | ||
** If a function is called on an STL container, you don't need to specify move semantics, the container implementation takes care of it. | ** If a function is called on an STL container, you don't need to specify move semantics, the container implementation takes care of it. | ||
| − | *:- No need for a function overload <code>void SomeFunc(std::string&& donor)</code> | + | *:- No need for a function overload of <code>void SomeFunc(std::string&& donor)</code>. |
| − | * | + | *:: if the default function is defined as <code>void SomeFunc(const std::string& str)</code>, <br />just use a single <code>void SomeFunc(std::string str)</code> that takes (in this example) a string by value |
*:- If an lvalue is passed as the argument, the string will be copied into the local variable | *:- If an lvalue is passed as the argument, the string will be copied into the local variable | ||
*:- If an rvalue is passed as the argument, the string will be move the data into the local variable | *:- If an rvalue is passed as the argument, the string will be move the data into the local variable | ||
Revision as of 03:51, 21 October 2019
In this video we learn about r-value reference and move semantics, which is perhaps the most important feature that was added in the C++11 update. This is going to allow us to manage and transfer our resources in a precise and efficient manner. It is sexy as fuck and I love it.
Topics Covered
- r-values and l-values
- r-value reference function overloading
- Move constructor and move assigment
- Rule of 5
-
std::move -
std::make_move_iterator
Video Timestamp Index
- Why are move semantics important and required? 0:23
- Example use case: adding objects to a container using
emplace_back - Instead of deep copying every element of the container when expanding the container capacity, move semantics allow you to direct pointers in the new container to existing blocks of memory
- Example use case: adding objects to a container using
- What is an rvalue (simple definition) 2:48
- rvalues are values that can only appear on the right hand side of an assignment operation (=), such as:
- - literals;
69 = 420;is invalid - - temporary return values of functions;
f() = 420;, where the signature offisint f();, is invalid
- lvalues are values that can appear on the left hand side of assignments; they can be assigned to, for example:
- -
x = 10;ifxhas been declared asint x; - -
obj.Get() = 69;if the member functionGet()returns a reference to an integer member variable - -
arr[3] = 10;ifarrhas been declared as an array of integers
- Why is the distinction between rvalues and lvalues useful? 4:57
- The ability to detect whether a function is called with an rvalue (rather than with an lvalue) is useful, because the function can freely mangle an rvalue as it will be destoyed after the function call anyway
- Overloading a function to take an rvalue reference using
&&6:11
- Using the
&&declarator in a function signature, e.g.void SomeFunc(SomeType&& donor) - This makes the compiler dispatch a function call to the overloaded version when an rvalue is passed
(note:&&was added in c++11;&is the familiar lvalue reference declarator)
- Using the
-
std::move(): Converting a variable into an rvalue reference 8:00
- This will allow the function to steal/use the resource(s) that are passed in
- This can be useful if you don't need those resources anymore
-
std::move(someVar)is shorthand (from the STL) for the static caststatic_cast<Type&&>(someVar) - All the STL containers are already wired up to be able to take advantage of "move semantics"
- - For example:
std::string a = std::move(b)will leaveb(also a std::string) empty after execution - - There is no magic to this call; all it does is pass
bas an rvalue reference, calling the overloaded constructor (the "move constructor"), and the overloaded assignment operator ("move assignment") ofstd::stringin which the contents ofbare passed over toaand removed fromb.
- Move constructor and Move assignment 10:07
- The STL containers will try to use move semantics as much as possible
- To benefit from this, any custom classes (or rather: any Type) will need to define a move constructor and a move assignment operator
-
std::movewill default to the normal (lvalue reference) constructor and copy assignment operator if their move (rvalue reference) counterparts are not defined
- Implementing the move constructor and move assignment operator in the
Surfaceclass of the Sprite drawing project 11:09
- Signature of the move constructor:
Surface(Surface&& donor) noexceptthat pilfers the data from the donor - Signature of the move assignnment:
const Surface& operator=(Surface&& rhs) noexcept
- Signature of the move constructor:
- The full story on implicit compiler definition of special member functions and the "Rule-of-5" 14:22
- If you implement any of the special member functions, you should implement all of them:
- - Destructor
- - Copy constructor
- - Copy assignment
- - Move constructor
- - Move assignment
- The behavior of the default move members and the "Rule-of-0" 15:17
- If a class only contain data members (Types) that handle their own RAII behavior (such as the STL containers), then you don't need to supply any move member functions
- If a function is called on an STL container, you don't need to specify move semantics, the container implementation takes care of it.
- - No need for a function overload of
void SomeFunc(std::string&& donor).- if the default function is defined as
void SomeFunc(const std::string& str),
just use a singlevoid SomeFunc(std::string str)that takes (in this example) a string by value
- if the default function is defined as
- - If an lvalue is passed as the argument, the string will be copied into the local variable
- - If an rvalue is passed as the argument, the string will be move the data into the local variable
- WORK-IN-PROGRESS
Note
If you are using Visual Studio 2017, you might notice that our move members are not being used during std::vector growth even after we have 'properly' implemented them. This is expected, so don't pay it much mind at the moment.
The reason for this is that std::vector will only use the move members if they are guaranteed not to throw any exceptions (as per the standard). This issue will be dealt with in Intermediate 22 (tutorial on exceptions). For your interest, you can enable the move optimization by declaring your move ctor as Surface( Surface&& ) noexcept. The same should be done for the move assignment and the destructor.
You might wonder why Chili had no problems in the video. In the video, Chili was using Visual Studio 2015, which does not conform to the standard in this point, so it uses the move members regardless of whether or not they are marked noexcept.
