Suppose I have a class with three static functions like this :
#include <vector>
#include <iostream>
using namespace std;
#include <thread>
class Employee
{
};
class client
{
public:
void Doprocessing()
{
//is this thread safe in c++11/14
static int i = CreateEmployee();
//does it look good to use static variable like this to make it thread safe?
static int k = ProcessEmploye();
}
private:
static int CreateEmployee()
{
static Employee * e = new Employee();
InsertEmployee(e);
return 1;
}
static int InsertEmployee(Employee *e)
{
vec.push_back(e);
return 1;
}
static int ProcessEmploye()
{
Employee* e = vec[0];
//do something with e
//...
//.
//Suppose 10 -20 lines
return 1;
}
static std::vector<Employee*> vec;
};
std::vector<Employee*> client::vec;
void func()
{
client cobj;
cobj.Doprocessing();
}
const int No_Of_Threads = 10;
int main() {
std::thread * threadpointer = new std::thread[No_Of_Threads];
std::srand(11);
for (int i = 0; i < No_Of_Threads; i++)
{
threadpointer[i] = std::thread(func);
}
for (int i = 0; i < No_Of_Threads; i++)
{
threadpointer[i].join();
}
delete[] threadpointer;
std::cout << " Good" << std::endl;
return 0;
}
My questions are :
1)If I use static int i = Somefunc() and no matter how bulky somefunc is, will be called once and will it be thread safe ?
2) if answer of 1) is yes, Does it look good to programers eyes to use static int i = SomeFunc() for above purpose.
Yes, it will be thread safe, but only since C++11. Static variables are initialized in thread safe way, they are often also called magic statics.
For more see here: http://en.cppreference.com/w/cpp/language/storage_duration#Static_local_variables
If multiple threads attempt to initialize the same static local variable concurrently, the initialization occurs exactly once (similar behavior can be obtained for arbitrary functions with std::call_once). Note: usual implementations of this feature use variants of the double-checked locking pattern, which reduces runtime overhead for already-initialized local statics to a single non-atomic boolean comparison.
Also - in your code you call CreateEmployee(); during initialization of static i, and CreateEmployee( also initializes a static variable. This should also be OK, you can find in standard following footnote:
The implementation must not introduce any deadlock around execution of the initializer.
As to your second question, from the code you have shown I dont see that it is OK to use static variable as a way to gain thread safety.
Are you aware that specifying a variable as static inside function body allows you to assign it only once? This means your CreateEmployee() will always return the same Employee instance.
来源：https://stackoverflow.com/questions/39487075/local-static-variable-initialization-is-thread-safe