c++ 静态变量

You will need to define the static variable, even if it is not initialized explicitly. That is what is missing in your code. You should have provided a simple example to reproduce the issue, but for your convenience I am providing one which works.

main.cpp

class Foo {
    public:
        static int si;
        static void bar();
};

int Foo::si = 0; // By default, it will be initialized to zero though.

void Foo::bar() {
     Foo::si = 10;
};

int main()
{
    Foo::bar();
    return 0;
}

===================================================================

The correct code is (.cpp):

#include "rtnamedinstance.h"

// Initialize static members
int RtNamedInstance::_nextInstanceNumber = 0;
QMutex RtNamedInstance::_syncObj(QMutex::Recursive);

RtNamedInstance::RtNamedInstance(QString instanceName)
{
    QMutexLocker locker(&_syncObj);

    // [... other code here ...]
}