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welcome to the copperspice youtube channel and thanks for joining us in this video we will discuss how variable initialization has become a bit more complicated in c plus what is initialization the process of initializing variables feels like something which should be simple to understand and use in computer languages like python and javascript there are very few rules and the way variables are initialized is rather straightforward in c plus this subject can get really messy very quickly all of the terminology is defined in the c plot standard if you can read it and apply the rules the examples on cpp reference can be confusing and it takes time and effort to work through the code it is important to remember the c plus plus standard is a living document which changes as definitions are refined the information in this talk is based on our best understanding as of c plus plus 20 and most of the examples will only require c plus plus 17. the first question developers should ask is why should variables be initialized in c plus accessing any variable or object which has not been properly initialized to a valid value will result in undefined behavior that seems like a good reason to ensure every variable and object has been initialized before its value is read since undefined behavior must be avoided in any correct c plus program initialization is your responsibility some of the objects created in a program will automatically have a valid initial value for the given data type some will not as the developer you need to know the basic rules there are two terms which sound very similar however they are fundamentally different the process of initialization occurs when a new object is declared and as part of the declaration the variable receives some value this value can be something you specified or it might be compiler generated this is where the terms get a bit tricky if the initialization code specifies a value the expression which provides this value is called an initializer there are several different ways to do initialization in c plus and most likely your code is already using several of them the idea we want to present is that there are other forms of initialization which may improve code readability and correctness the simplest form of initialization is called default initialization this form occurs when you declare a variable without providing any initializer the value for a default initialized object depends on two factors the data type of the object and its lifetime in this example function all of the variables are being declared locally primitive types are default initialized to an unknown value according to the c plus plus standard this means var a has an indeterminate value it might be zero but it is more likely to be a random garbage value which may not actually be a valid integer reading the value of var a or var c in this function will result in undefined behavior if the object being declared is a class or a struct the default constructor will be called if one exists in our example the default constructor for std string will be used to initialize var b to a valid empty string if a variable declaration is static or has global lifetime then the rules for default initialization change since var d is static the value will be initialized to 0 at the beginning of the program even though int is a primitive type another way to look at this is that var d is default initialized using zero initialization for var e which has a type of std string the default initialization does not change when the static keyword is added for objects which live for the duration of the program the behavior of default initialization will also use zero initialization just like for static declarations brace initialization is a term which is commonly used in the c plus community since many of the modern forms of initialization can use curly braces we felt it was a value to mention this term does not actually appear in the c-plus plus standard using the phrase brace initialization is not harmful just be aware it does not identify one of the defined forms of initialization we have listed the five forms of initialization which can use curly braces value initialization will almost always use curly braces and there must be nothing inside in the declaration the variable will be initialized with an empty or zero value whatever is appropriate for the given data type if the object being declared is a class or struct the rules are a bit more complex if the class has a default constructor it will be called however before the default constructor executes the data members of the object may be zero initialized by the compiler in our first example var1 will be set to an integer with a value of 0. for var2 the default constructor for std string will be called and the initial value will be an empty string in the third example the initial value of var 3 will be a null pointer the return statement is pretty interesting it may not be obvious but this line of code also uses value initialization and 0.0 will be returned for this line of code you can use either parentheses or curly braces and the result will be the same with the other three examples in this function you must use curly braces to prevent the initialization from being parsed as a function declaration to be clear we do not want to declare a function named var1 but rather a variable of type int when c plus plus programmers talk about initialization the most common form they think about is direct initialization typically parentheses are used and there must be one or more values curly braces can also be used however if there is more than one value supplied with curly braces it is not actually direct initialization but rather list initialization we will show some examples of this a bit later on in our first two examples here the variables width and height use direct initialization even though one uses parentheses and the other uses curly braces for the third example we are using direct initialization to call the constructor for std pair this may be obvious however it is worth noting there is a significant difference between copy assignment and copy initialization these examples are all copy initialization at first glance they do sort of look like assignment operations however since the data type is specified these are declarations when a variable is declared and the initial value is specified using an equal sign it is copy initialization as a minor point move assignment is a valid operation which was added in c plus plus 11. the standards committee did not need to add move initialization because it is unnecessary the process of copy initialization does not actually do what we think of as a copy there is no actual copy happening here but rather an instantiation prior to c plus 17 a class like std atomic could not be initialized using copy initialization since it was not supported the initializations for the variables count a and count b produce the same result in a c plus 17 or c plus 20 compiler list initialization is one we believe adds the most benefit for developers this form of syntax always uses curly braces our first three declaration examples under a all use an equal sign even though the variable delta is just an int and found is simply a bool they are each being initialized using a list containing one element the variable word is declared with the data type of std string which is a class the list initialization has an initializer of five character values since this class has a constructor which takes an initializer list these values are received as a list and word is assigned the value of tiger if the std string class did not have an initializer list constructor the compiler would have looked for a constructor which took five separate care parameters if neither of these constructors were available it would be a compile error since using a list initialization is very convenient and tends to improve readability some developers are tempted to convert all parentheses used for initialization into curly braces we have read blogs which proclaim using list initialization everywhere is a good way to prove you are writing modern c plus code this may be true however developers all need to know about the errors which can be introduced by doing a quick search and replace in example b we are initializing x and y which both have the data type of std vector since x uses curly braces and has more than one argument this is list initialization the vector x will contain two int elements with the values 5 and 10. however vector y uses parentheses so direct initialization will occur in this example the compiler will call the constructor which takes two integer parameters the first parameter is the count which defines how many elements should be created the second parameter is the value for each element so y will contain two integer elements both of which have the value five clearly using list initialization versus direct initialization for std vector produces different results we have explained most of the forms of initialization available in c plus and shown the fundamental principles however there are more areas to cover such as aggregate initialization and how it interacts with list initialization there are also some fascinating corner cases some of which you may want to use and others which should be avoided in a future video we will explain more of initialization and talk about ways to reason through your code for more information about copper spice please visit our website at www.copperspice.com thanks for watching we hope you found the content of value if you have any questions or feedback feel free to leave a comment on this video or send us an email please make sure you subscribe to our youtube channel and come back in a few weeks for our next video