Quote from http://msdn2.microsoft.com/en-us/library/s1sb61xd.aspx: "Assigning to a static local variable is not thread safe and is not recommended as a programming practice."
Although this isn't very concise wording (IIRC, the standard says that it should work for literals assigned to PODs) it seems clear that it won't work for non-PODs...
The example below the warning clearly show up what they mean, and it's really a concurrency situation where two threads access the same object. So, yes, this is a bad practice, and it is clearly not thread-safe, even for POD: // static3.cpp // compile with: /EHsc #include <iostream> using namespace std; struct C { void Test(int value) { static int var = 0; if (var == value) cout << "var == value" << endl; else cout << "var != value" << endl; var = value; } }; int main() { C c1; C c2; c1.Test(100); c2.Test(100); } Output var != value var == value