Advanced C++: Understanding rvalue and lvalue

expression must be a modifiable lvalue
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hello everyone welcome to C++ advanced today we are going to talk about our value and our value our value and our value are not something that you learn into very often some people has programmed the C++ for years and they still dont have a good understanding of what our value and our values are so the question to ask is why do I care our value and L value are actually two very important concept in C++ core language having a good knowledge of them will help you understand C++ construct and to explain some similar strange behavior of C++ it also can help you to decipher compiler errors and warnings because these two terms appears quite often in the compiler messages a second reason that you should care about them is C++ 11 introduced a new feature called our value reference if you dont have a good grasp of our value and our value it will be impossible for you to learn our value reference so what are the error values and our values it is hard to give a rigorous definition of what they are so instead here Im giving you a simplified definition which is generally accepted and it should serve you well in 99% of the time l value is an object that occupies some identifiable location in memory so the key word here is its something in memory not something in the register and secondly it is it has identifiable address our value is defined by exclusion any object that is not a L value its our value now lets look at some L value examples I have an integer I I is

a L value Y it is our L value because I can get its address with am % and assign the edges to a integer pointer so its edges it not only has a address and its address is identifiable and that the content of that edges can be modified by assigning I to a different valuable second example is I have a class dog and then I will create a dog D 1 and D 1 is a l value of a user-defined the type a class it is fair to say that most of the variables in C++ code are l values now lets look at some other value examples I have integer X equal to 2 as we said X is a l value and the 2 is our value X equal to I plus 2 I plus 2 is our value why are they our values because if I try to get their edges like this the compiler will error out and I also cannot assign a different value to them so there are values another example I have a dog D 1 and D 1 equal to talk and dog is our value of user defined type class now lets look at function I have a function sum which takes an integer x and y and return the sum of X Y and then I have I equal to sum 3 for some 3/4 is the r-value how in summary what are the are values in these examples two is our value I plus two is I value dog and the Sun and also X plus y is also our value and what are the

L values X hi t1 thats it oh I have another one P now lets look at reference I assume you are already familiar with the reference in C++ here I create an integer I and initialize the integer reference are with I this reference is actually should be called L value reference because R is a reference to our L value I and if I initialize the reference our ways are value of five then the compiler will error out and the only exception for this is if R is a constant reference then it can be assigned with five although you should you really should consider this is a shortcut of two step operation step one our L value is created with five and step number two the reference R is initialized with this temporary l value now lets look at the same idea with the function I have a function Square which takes l value reference X and returns X square when I invoke the function square I I is the integer and it is okay if I invoke the function square for T then it is an error because for T is not a l value its an R value how can I make the fungus function square for T work the workaround is I can change the parameter of the square function into a Const integer reference and as weve seen in previous example a Const individual reference can be initialized with our value so the square for T will work and the square I still will work what confuses most of the people in the transforming between the L value and our value an

L value can be used to create our value in this example I have a L value I and I can create our value with I plus 2 and that second example is in GG X equal to I now what is I is the is it our value or L value I is obviously our L value because it has edges and I address is identifiable however in this context I is implicitly transformed into our value so our L value can be implicitly transformed into our value however the other way around is not valid and our value cannot be implicitly transformed into a l value and our value should be explicitly used to create our l value heres an example of using R value to create our value I have an integer array V and the V Plus 2 is our R value however the dereference of V plus 2 is a l value which can be assigned with a different value instead of continue to talk about what the our values and our values are lets talk about what they are not there are some misconception that needs to be cleared up miss concept one function or operator always yields our values this conclusion can be easily drawn from the examples of where that weve seen so far like I equal I plus three is our value some 3/4 is our value however this conclusion is not right and here is my counter example I have a function foo which returns an integer reference and in the function I return the global valuable by reference so the return value from the function foo is a l

write code foo equal to 15 this will compile some of you may say well this is a weird-looking code I never write code like this well point taken then how about this example array 3 equal to 15 in this example Im invoking operator square bracket on array and operate a square bracket almost always generates L value misconcept number 2 our values are modifiable people usually get this idea from the laksa definition of our value in C language in C L value is defined as a value suitable for left-hand side of assignment however this definition is no longer true in C++ because C++ has a Const and a Const although is a l value it is not modifiable misconcept number three our values are not modifiable this seems to be pretty obvious in our examples I plus three is our value and its not modifiable some three four is our value and its not modifiable however this conclusion is only true for the built-in types it is not true for user-defined the type or class for example I can call the default constructor of dog to create a dog and then invoke the function Park on the dog the function Park could change the state of the stock so this our value dog is modifiable a summary if you cannot remember everything that Ive talked about our value and L value please do remember the summary number one every C++ expression yields either our value or al value number two if an expression has an identifiable memory address it is a l value otherwise its already thats all for todays session thank you for watching bye bye

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