When are global variables initialized before entering the main function?

Global variables ,static Variable initialization time

First , Static local variables are the same as global variables , The data is stored in the global area Memory distribution , So before the main program , The compiler has allocated memory for it , But in C and C++ The initialization node of static local variables in is a little different . stay C in , Initialization occurs before code execution , After allocating memory in the compilation phase , It will initialize , So we see in C Variables cannot be used to initialize static local variables in language , At the end of the program , The global memory where the variable is located will be fully recycled . And in the C++ in , When initializing, it will be initialized when the relevant code is executed , Mainly due to C++ After introducing objects , To initialize, the corresponding constructor and destructor must be executed , In the constructor or destructor, you often need to do some specific operations in the program , It's not just about allocating memory . therefore C++ The standard is that global or static objects are constructed only when they are first used , And pass atexit() To manage . At the end of the program , According to the order of construction, they are destructed one by one in the opposite direction . So in C++ You can use variables to initialize static local variables .
Global variables 、 The static variables of the file field and the static member variables of the class are in main Allocate memory and initialize during previous static initialization ; Local static variable （ Generally, it is a static variable in a function ） Allocate memory and initialize on first use . The variables here contain objects of built-in data types and custom types .
according to C++ standard , The initialization of global variables should be in main Complete before function execution , Common sense is beyond doubt , But this statement is a little vague ,main When exactly is it before the function is executed ？ Is it compile time or run time ？ The answer is both compile time , There may also be runtime (seriously), From the language level , The initialization of global variables can be divided into the following two stages （c++11 N3690 3.6.2)：

1. Static variables are initialized at compile time , The assignment of variables is carried out when the function or program is running .
2. Static variables are initialized only once , However, the value of a static variable can be modified many times by assignment .
3. Global and static variables When entering main Initialized before