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Like most things, there are advantages and disadvantages to credit cards. Knowing some of these can help you decide if you do or do not want to use credit cards.
Credit cards can make life easier and be a great tool, but if they aren't used wisely they can become a huge financial burden. If you do decide to use credit cards, remember these simple rules:
Learn more about how to apply for credit cards in the next section
A declaration introduces a name – an identifier – to the compiler. It tells the compiler “This function or this variable exists somewhere, and here is what it should look like.” A definition, on the other hand, says: “Make this variable here” or “Make this function here.” It allocates storage for the name. This meaning works whether you’re talking about a variable or a function; in either case, at the point of definition the compiler allocates storage. For a variable, the compiler determines how big that variable is and causes space to be generated in memory to hold the data for that variable. For a function, the compiler generates code, which ends up occupying storage in memory.
A definition can also be a declaration. If the compiler hasn’t seen the name xbefore and you define int x;, the compiler sees the name as a declaration and allocates storage for it all at once.
A function declaration in C and C++ gives the function name, the argument types passed to the function, and the return value of the function. For example, here is a declaration for a function called func1( ) that takes two integer arguments (integers are denoted in C/C++ with the keyword int) and returns an integer.
int func1(int, int);
A variable declaration tells the compiler what a variable looks like. It says, “I know you haven’t seen this name before, but I promise it exists someplace, and it’s a variable of X type.”
In a function declaration, you give a type (the return value), the function name, the argument list, and a semicolon. That’s enough for the compiler to figure out that it’s a declaration and what the function should look like. By inference, a variable declaration might be a type followed by a name. For example:
int a;
could declare the variable a as an integer, using the logic above. Here’s the conflict: there is enough information in the code above for the compiler to create space for an integer called a, and that’s what happens. To resolve this dilemma, a keyword was necessary for C and C++ to say “This is only a declaration; it’s defined elsewhere.” The keyword is extern. It can mean the definition is external to the file, or that the definition occurs later in the file.
Declaring a variable without defining it means using the extern keyword before a description of the variable, like this:
extern int a;
Coming to your question, yes header files contains declarations but also some definitions. It mostly contains preprocessor directives like #define, #ifdef etc which the preprocessor step (before the compilation) will expand and allow functions and variables available for the compiling module.