示例：

#ifndef TEST_H
#define TEST_H


class test
{
public:
    static test * GetINSTANCE();
    void print();
private:
    test();
};

#endif // TEST_H

#include "test.h"
#include <QMutex>
#include <QDebug>
test::test()
{

}

test *test::GetINSTANCE()
{
    static test * inst = nullptr;
    static QMutex mutex;
    mutex.lock();
    if(!inst)
    {
        inst = new test;
    }
    mutex.unlock();
    return inst;
}

void test::print()
{
    static int m = 0;
    m++;
    qDebug()<<__FILE__<<"["<<__LINE__<<"]"<< m;
}

补充：

这样写也可以：

    static T * GetINSTANCE()
    {
        static QScopedPointer<T> inst;
        static QMutex mutex;
        mutex.lock();
        if(!inst)
        {
            inst.reset(new T());
        }
        mutex.unlock();
        return inst.data();
    }

    test::GetINSTANCE()->print();
    test::GetINSTANCE()->print();
    test::GetINSTANCE()->print();

..\try\test.cpp [ 26 ] 1

..\try\test.cpp [ 26 ] 2

..\try\test.cpp [ 26 ] 3

使用类模板进行简单的优化，使代码扩展性更好一些：

#ifndef TEST_H
#define TEST_H

#include <QMutex>

template<typename T>
class Singleton
{
public:
    static T * GetINSTANCE()
    {
        static T * inst = nullptr;
        static QMutex mutex;
        mutex.lock();
        if(!inst)
        {
            inst = new T();
        }
        mutex.unlock();
        return inst;
    }
protected:
    Singleton() = default;
};


class test:public Singleton<test>
{
    friend class Singleton<test>;
public:
    void print();
private:
    test() = default;
};
#endif // TEST_H

#include "test.h"
#include <QDebug>

void test::print()
{
    static int m = 0;
    m++;
    qDebug()<<__FILE__<<"["<<__LINE__<<"]"<< m;
}

    test::GetINSTANCE()->print();
    test::GetINSTANCE()->print();
    test::GetINSTANCE()->print();

..\try\test.cpp [ 11 ] 1

..\try\test.cpp [ 11 ] 2

..\try\test.cpp [ 11 ] 3