Local Variables in Visual Studio .NET

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Local Variables
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As you know, variables that are declared inside a function are called local variables However, C++ supports a more subtle concept of the local variable than you have previously seen In C++, variables can be localized to a block That is, a variable can be declared inside any block of code and is then local to it (Remember, a block begins with an opening curly brace and ends with a closing curly brace) In reality, variables local to a function are simply a special case of the more general concept A local variable can be used only by statements located within the block in which it is declared Stated another way, a local variable is not known outside its own code block Thus, statements outside a block cannot access an object defined within the block One of the most important things to understand about local variables is that they exist only while the block of code in which they are declared is executing This means that a local variable is created upon entry into its block and destroyed upon exit Because a local variable is destroyed upon exit from its block, its value is lost The most common code block is the function In C++, each function defines a block of code that begins with the function s opening curly brace and ends with its closing curly brace A function s code and data are private to that function, and cannot be accessed by any statement in any other function, except through a call to that function (It is not possible, for instance, to use a goto statement to jump into the middle of another function) The body of a function is hidden from the rest of the program and, unless it uses global variables, it can neither affect nor be affected by other parts of the program Thus, the contents of one function are completely separate from the contents of another Stated another way, the code and data that are defined within one function cannot interact with the code or data defined in another function, because the two functions have a different scope Because each function defines its own scope, the variables declared within one function have no effect on those declared in another even if those variables share the same name For example, consider the following program:
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Functions, Part One: The Fundamentals
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#include <iostream> using namespace std; void f1(); int main() { char str[] = "this is str in main()"; cout << str << '\n'; f1(); cout << str << '\n'; return 0; } void f1() { char str[80]; cout << "Enter something: "; cin >> str; cout << str << '\n'; }
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A character array called str is declared twice here, once in main( ) and once in f1( ) The str in main( ) has no bearing on, or relationship to, the one in f1( ) The reason for this is that each str is known only to the block in which it is declared To confirm this, try running the program As you will see, even though str receives a string entered by the user inside f1( ), the contents of str in main( ) remain unchanged The C++ language contains the keyword auto, which can be used to declare local variables However, since all non-global variables are, by default, assumed to be auto, it is virtually never used Hence, you will not see it used in any of the examples in this book However, if you choose to use it, place it immediately before the variable s type, as shown here:
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It is common practice to declare all variables needed within a function at the beginning of that function s code block This is done mainly so that anyone reading the code can easily determine what variables are used However, the beginning of the function s block is not the only place where local variables can be declared A local variable can be declared anywhere, within any block of code A variable declared within a block is local to that block This means that the variable does not exist until the block is entered and is destroyed when the block is exited Furthermore, no code outside that block including other code in the function can access that variable
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To understand this, try the following program:
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