[C ++ Note] 2. Variable declaration and definition, and variable Declaration

[C ++ Note] 2. Variable declaration and definition, and variable Declaration

It is impossible to put all C ++ programs in one file. Therefore, C ++ supports the separate compilation mechanism, which allows programs to be divided into several files, each file can be compiled independently. This shows that when your program has many files, you do not need to recompile all the files after modifying one of the files. You only need to compile and modify the files, then link them all.

How does C ++ support separate compilation? Separate the Declaration and definition.

Declaration (declaration) makes the name known to the program. If a file wants to use a name defined elsewhere, it must contain the declaration of the name.

Definition is responsible for creating entities associated with names.

A declaration statement is composed of a basic data type and a declarator list followed by it.

You also need to apply for a bucket or assign an initial value to the variable.

If you want to declare a variable rather than define it, assign an initial value to the variable marked with the keyword extern before the variable name, but this will offset the effect of extern. If an extern statement contains an initial value, it is no longer a declaration but a definition.

Extern int I; // declare I instead of defining iint j; // declare and define j

In the function body, if you try to initialize a variable marked by the extern keyword, an error is thrown.

// Error int main () {extern int I; int I = 10; return 0 ;}

Variables can be defined only once, but can be declared multiple times.

// You can obtain extern int I; int main () {int I = 10; return 0 ;}

To use the same variable in multiple files, you must separate the Declaration and definition. However, the definition of a variable must appear only in one file. Other files that use the variable must be declared, but cannot be defined repeatedly.

In addition, C ++ checks the type at the compilation stage. Therefore, C ++ is also known as statically typed. The more complex the program is, the more the static type persistence helps to identify problems. Then, the premise is that the compiler must know the type of each object, which requires that we must declare its type before using a variable.

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