The set of all global variables is known as the global environment or global state. For functions, definitions supply the function body To understand how external variables relate to the extern keyword, it is necessary to know the difference between defining and declaring a variable
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When a variable is defined, the compiler allocates memory for that variable and possibly also initializes its contents to some value.
The most common variable types in c++ are local variables inside a function or block, and temporary variables
[6] the common feature about automatic variables is that they have a lifetime that is limited to the scope of the variable. Data segment in computing, a data segment (often denoted.data) is a portion of an object file or the corresponding address space of a program that contains initialized static variables, that is, global variables and static local variables. A global variable is a variable declared in the main body of the source code, outside all functions, while a local variable is one declared within the body of a function or a block. A third way is by declaring and defining a variable as being constant
A global variable or static variable can be declared (or a symbol defined in assembly) with a keyword qualifier such as const, constexpr, constant, or final (when it is applied to primitive types), meaning that its value will be set at compile time and should not be. A second use case would be multiple threads accumulating information into a global variable To avoid a race condition, every access to this global variable would have to be protected by a mutex. For variables, definitions assign values to an area of memory that was reserved during the declaration phase
