几个storage class keyword(C++)

auto, static, register and extern are  storage class specifiers, we can only use one of these four in a declaration.

Register: The variable is to be stored in a machine register, instead of memory (for example, instead of the stack) . This is useful if we want to use the variable very frequently. Practically, the compiler will do its own optimization and the keyword will probably have no effect.

Static: The variable will be in file scope, so it is accessible within the compilation unit (Compare:  a global variable can be accessed from any compilation with external linkage, and a local variable can be accessed only within block).

Extern:  when used with a string("C"/"C++"), extern specifies that the linkage conventions of another language are being used for the declarator(s). When modifying a variable, extern specifies that the variable has static duration.

Auto: The variable will be in local block-scope, since this is by default any way, auto is
basically not necessary to use. However, in  C++11, auto has new meaning:  the definition of a 

variable with an explicit initialization can use the auto keyword. 

Example: 

std::list<int> a; 

// fill in a 

for (auto it =a.begin(); it!= a.end(); ++it) { 

// Do stuff here 

}