C++ Variable Assignment and Initialization
1. Define#
int x; // define an integer variable named x
int y, z; // define two integer variables, named y and z
2. Variable Assignment#
After a variable has been defined, you can give it a value (in a separate statement) using the = operator. This process is called copy assignment (or just assignment) for short:
int width; // define an integer variable named width
width = 5; // copy assignment of value 5 into variable width
3. Initialization#
Initialization in C++ is surprisingly complex, so we’ll present a simplified view here.
There are 6 basic ways to initialize variables in C++:
int a; // no initializer (default initialization)
int b = 5; // initializer after equals sign (copy initialization)
int c(6); // initializer in parenthesis (direct initialization)
// List initialization methods (C++11) (preferred)
int d{7}; // initializer in braces (direct list initialization)
int e = {8}; // initializer in braces after equals sign (copy list initialization)
int f {}; // initializer is empty braces (value initialization)
3.1. Default Initialization#
When no initialization value is provided (such as for variable a above), this is called default initialization. In most cases, default initialization leaves a variable with an indeterminate value.
int a; // no initializer (default initialization)
3.2. Copy Initialization#
int width = 5; // copy initialization of value 5 into variable width
Copy initialization is also used whenever values are implicitly copied or converted, such as when passing arguments to a function by value, returning from a function by value, or catching exceptions by value.
3.3. List Initialization#
The modern way to initialize objects in C++ is to use a form of initialization that makes use of curly braces: list initialization (also called uniform initialization or brace initialization). List initialization comes in three forms:
int width {5}; // direct list initialization of value 5 into variable width
int height = {6}; // copy list initialization of value 6 into variable height
int depth {}; // value initialization (see next section)
List initialization has an added benefit: it disallows “narrowing conversions”. This means that if you try to brace initialize a variable using a value that the variable can not safely hold, the compiler will produce an error. For example:
int width { 4.5 }; // error: a number with a fractional value can't fit into an int
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