Question

I discovered the source of a really insidious bug today. It’s all safely fixed now, but I’d like to understand why when I execute the following code:

using namespace System;

ref class EvilClass
{
public:
    EvilClass()
    {

    }

    void Print()
    {
        static bool enablePrint = false;
        if( enablePrint )
        {
            Console::WriteLine("PrintEnabled");
        }
        else
        {
            Console::WriteLine("PrintDisabled");
        }
        enablePrint = true;
    }

};


int main(array<System::String ^> ^args)
{
    Console::WriteLine(L"Hello World");

    EvilClass^ ec = gcnew EvilClass();

    ec->Print();
    ec->Print();

    delete ec;
    ec = nullptr;

    ec = gcnew EvilClass();

    ec->Print();
    ec->Print();

    delete ec;
    ec = nullptr;

    return 0;
}

…I get the following:

Hello World
PrintDisabled
PrintEnabled
PrintEnabled
PrintEnabled

I had always assumed that the static would only persist between calls to the same instance of a class?

Answer 1

Your assumption is incorrect. A function-scoped static variable is very similar to a global variable. There is only one global instance of it. More details here: What is the lifetime of a static variable in a C++ function?