hiding and exposure

static, inline, extern, thread_local

static

static in class

when the keyword static is used to the class member in c++, it means the static member belongs to the class and can be accessed by all the instantiated objects. static func can only access other static member because they don’t know the this pointer that points to each object.

class Myclass {
    private:
        static int cnt;
        const static int tot = 10;
    public:
        static void Incr(int add) {
            cnt += add;
        }
};
int Myclass::cnt = 0;   // static  mem var should be initialized out of the class and explicitly

static variables are located in the data segement of address space, so their lifetime is same as the programe.

static acts on general func

when the static keyword acts on the general func or member, it limits the visibility only to the translation unit (object file compiled from .c/.c++ file and its header)。

// a.cpp
#include <iostream>
void test() {
    std::cout << "this is test is a.cpp\n";
}
```cpp
// b.cpp
#include <iostream>
using namespace std;
static void test() {
    std::cout << __PRETTY_FUNCTION__ << std::endl;
    std::cout << "this is test in b.cpp" << std::endl;
}

// main.cpp
#include <iostream>
#include <type_traits>
using namespace std;

void test();

int main() {
    test();
}

(base) ➜  static git:(master) ✗ g++ -std=c++17 main.cpp a.cpp b.cpp -o main 
(base) ➜  static git:(master) ✗ ./main  
this is test is test.cpp

if don’t compile a.cpp, symbol not found error will be emitted, even though there is a specifed test in b.cpp

(base) ➜  static git:(master) ✗ g++ -std=c++17 main.cpp b.cpp -o main          
Undefined symbols for architecture x86_64:
  "test()", referenced from:
      _main in main-07d8dc.o
ld: symbol(s) not found for architecture x86_64
clang++: error: linker command failed with exit code 1 (use -v to see invocation)

problems with static

Because static var is local to the translation unit, different units can have their own static var.

#include "test1.h"
#include "test2.h"

int main(){
        func1();
        func1();
        func2();
        func2();
        func1();
        func2();
        return 0;
}

// test1.h
#ifndef __TEST1_H
#define __TEST1_H
#include "static.h"
#include <iostream>
void func1();
#endif

// test1.cpp
#include <iostream>
void func1(){
        var += 10;
        std::cout << "in test1::func1 : var = " << var << std::endl;
        return;
}

// test2.h
#ifndef __TEST2_H
#define __TEST2_H
#include <iostream>
void func2();
#endif

// test2.cpp
#include <iostream>
#include "static.h"

void func2(){
        var += 1;
        std::cout << "in test2::func2 : var = " << var << std::endl;
        return;
}

#ifndef __STATIC_H
#define __STATIC_H
static int var = 1;
#endif

(base) ➜  static_var_in_header git:(master) ✗ g++ -std=c++17  main.cpp test2.cpp test1.cpp -o main  
(base) ➜  static_var_in_header git:(master) ✗ ./main  
in test1::func1 : var = 11
in test1::func1 : var = 21
in test2::func2 : var = 2
in test2::func2 : var = 3
in test1::func1 : var = 31
in test2::func2 : var = 4

func1 and func2 operate on its own var

what is a static func in c

extern

inline