（1）代码来自 qrandom.h ，结合官方的注释：

#ifndef QRANDOM_H
#define QRANDOM_H#include <QtCore/qalgorithms.h>
#include <algorithm>    // for std::generate
#include <random>       // for std::mt19937#ifdef min
#  undef min
#endif
#ifdef max
#  undef max
#endifQT_BEGIN_NAMESPACE  //说明本类直接定义在 QT 的命名空间class QRandomGenerator //Note: All functions in this class are reentrant.
{// restrict the template parameters to// unsigned integers 32 bits wide or larger// 为 true 时要求模板参数 UINT 在 32bit 以上template <typename UInt> //定义模板变量，本值是布尔量using IfValidUInt = typename std::enable_if<std::is_unsigned<UInt>::value &&sizeof(UInt) >= sizeof(uint), bool>::type;private: //私有部分会包含重要的数据成员的定义//typedef  QIntegerForSizeof<void *>::Signed  qptrdiff;Q_CORE_EXPORT quint64 _fillRange(void *buffer, qptrdiff count);//应该是从 buffer 地址开始填充 count 个随机数struct InitialRandomData { //本类用作下面的友元函数的返回值类型//typedef QIntegerForSizeof<void *>::Unsigned quintptr;quintptr data[16 / sizeof(quintptr)]; //sizeof(quintptr) = 8};friend InitialRandomData qt_initial_random_value() noexcept; //友元函数friend class QRandomGenerator64 ; //友元类struct SystemGenerator          ; //类声明struct SystemAndGlobalGenerators;using RandomEngine = std::mersenne_twister_engine< //模板别名quint32, 32,624,397,31,0x9908b0df,11,0xffffffff,7,0x9d2c5680,15,0xefc60000,18,1812433253>;union Storage //定义了类中类{uint dummy ;           //本类的数据成员
#ifdef Q_COMPILER_UNRESTRICTED_UNIONS //if分支有效RandomEngine twister;  //本类的成员变量RandomEngine & engine()       { return twister; }const RandomEngine & engine() const { return twister; }
#else                                 //else分支作废std::aligned_storage<sizeof(RandomEngine), alignof(RandomEngine)>::type buffer;RandomEngine &engine() { return reinterpret_cast<RandomEngine &>(buffer); }const RandomEngine &engine() const { return reinterpret_cast<const RandomEngine &>(buffer); }
#endifstatic_assert(std::is_trivially_destructible<RandomEngine>::value,"std::mersenne_twister not trivially平凡地 destructible as expected");constexpr Storage(); //本类的默认构造函数};Storage storage; //本随机类的数据成员uint    type   ; //又一个数据成员public://Initializes this QRandomGenerator object with the value seedValue//as the seed. Two objects constructed or reseeded with the same seed value//will produce the same number sequence.QRandomGenerator(quint32 seedValue = 1)  //本类的有参构造函数: QRandomGenerator(&seedValue, 1) {} //带默认参数，也可以作为无参构造函数//Initializes this QRandomGenerator object//with the values found in the array seedBuffer as the seed.//Two objects constructed or reseeded with the same seed value//will produce the same number sequence.//可以这么用： uint t[3]={1,2,3};  //此处必须是无符号数组， int 数组会报错//      QRandomGenerator ma(t);template <qsizetype N> //普通类型的模板参数，必须给值的QRandomGenerator(const quint32 (&seedBuffer)[N]) //形参是整型数组: QRandomGenerator(seedBuffer, seedBuffer + N) {}//Initializes this QRandomGenerator object//with len values found in the array seedBuffer as the seed.QRandomGenerator(const quint32 * seedBuffer, qsizetype len): QRandomGenerator(seedBuffer, seedBuffer + len) {}Q_CORE_EXPORT QRandomGenerator(std::seed_seq & sseq) noexcept;//构造函数//Initializes this QRandomGenerator object//with the values found in the range from begin to end as the seed.Q_CORE_EXPORT QRandomGenerator(const quint32 *begin, const quint32 *end);//本类的静态成员函数，返回一个指针，指向随机数生成器对象。英文注释讲解略//返回一个指向共享ORandomGenerator的指针，//该共享ORandomGenerator总是使用操作系统提供的工具来生成随机数。//系统设施在至少以下操作系统上被认为是加密安全的:苹果操作系统(Darwin)、BSD、//Linux、Windows。在其他操作系统上也可能出现这种情况。//他们也可能支持一个真正的硬件随机数发生器。//因此，此函数返回的ORandomGenerator不应用于批量数据生成。//相反，可以用它从<random>头部种子化ORandomGenerator或一个随机引擎。//此函数返回的对象是线程安全的，可以在任何线程中使用，无需锁定。//它也可以被复制，生成的 ORandomGenerator也会访问操作系统设施，但它们不会生成相同的序列。static inline Q_DECL_CONST_FUNCTION QRandomGenerator * system();//老师说不要用这个函数大量生成随机数据，只用来生成种子即可。！！！！++++//返回一个共享的QRandomGenerator的指针，该指针使用securelySeeded()进行初始化。//此函数应用于创建随机数据，//而无需为特定用途创建昂贵的securely-seeded ORandomGenerator//或存储较大的 QRandomGenerator对象。//对这个对象的访问是线程安全的，因此可以在任何线程中使用，无需锁定。//该对象也可以被复制，而复制产生的序列将与共享对象产生的序列相同。//但是，请注意，如果有其他线程访问全局对象，这些线程可能会在不可预测的间隔内获取样本。static inline Q_DECL_CONST_FUNCTION QRandomGenerator * global();//返回指针//老师说，一般情况用这个静态函数就足够了。！！！！！++++++//返回一个新的 QRandomGenerator 对象，该对象通过 QRandomGenerator:system()//安全地进行了初始化。此函数将为 ORandomGenerator 使用的算法获取理想的种子大小，//因此是创建将保留-段时间的新 ORandomGenerator 对象的推荐方法。//考虑到安全启动确定性引擎所需的数据量，这个函数有些昂贵，//不应用于QRandomGenerator 的短期使用//(使用它生成少于 2600 字节的随机数据实际上是一种资源浪费)。//如果使用不需要那么多数据，可以考虑使用 ORandomGenerator::global()//而不是存储一个 ORandomGenerator 对象。static inline QRandomGenerator securelySeeded();  //返回生成器对象// copy constructor & assignment operator (move unnecessary)Q_CORE_EXPORT QRandomGenerator(const QRandomGenerator &other);//copy 构造函数与 copy 赋值运算符函数Q_CORE_EXPORT QRandomGenerator & operator=(const QRandomGenerator & other);friend Q_CORE_EXPORT bool //声明了全局友元函数operator==(const QRandomGenerator &rng1, const QRandomGenerator &rng2);friend bool operator!=(const QRandomGenerator &rng1,const QRandomGenerator &rng2  ){return !(rng1 == rng2);}//Generates a 32-bit random quantity数量 and returns it.//调用了本类的私有成员函数 _fillRange()以实现本函的功能quint32 generate() { return quint32(_fillRange(nullptr, 1));  }//Generates a 64-bit random quantity and returns it.quint64 generate64(){return _fillRange(nullptr, sizeof(quint64) / sizeof(quint32));}//Generates one random qreal in the canonical range [0, 1)//(that is, inclusive of zero and exclusive of 1).double generateDouble(){// IEEE 754 double precision has: 双精度数值，共 64 bit 8字节//   1 bit      sign//  10 bits     exponent  指数部分//  53 bits     mantissa  有效数字// In order for our result to be normalized in the range [0, 1), we// need exactly 53 bits of random data. Use generate64() to get enough.quint64     x = generate64();quint64 limit = Q_UINT64_C(1) << std::numeric_limits<double>::digits;x >>= std::numeric_limits<quint64>::digits - //这是个减法运算std::numeric_limits<double> ::digits ;return double(x) / double(limit);}//Generates 32-bit quantities and stores them//in the range between begin and end.// API like std::seed_seqtemplate <typename ForwardIterator>void generate(ForwardIterator begin, ForwardIterator end){std::generate(begin, end, [this]() { return generate(); });}void generate(quint32 *begin, quint32 *end){_fillRange(begin, end - begin);}//generate 与 bound 差不多，前者的取值不受限制，后者有突出边界上限的意思。//Generates one random double in the range between 0 (inclusive) and//highest (exclusive).//This function is equivalent to and is implemented as://     return generateDouble() * highest; 此行是本函数的实现原理//If the highest parameter is negative,//the result will be negative too;//if it is infinite or NaN,//the result will be infinite or NaN too (that is, not random).double bounded(double highest) //形参是 double 类型{return generateDouble() * highest;}//typedef unsigned int quint32;quint32 bounded(quint32 highest) //形参是 32 位无符号整数{quint64 value = generate();value *= highest;            //先乘法后除法才不会损失精度value /= (max)() + quint64(1);return quint32(value);}//Generates one random 32-bit quantity in the range between 0 (inclusive)//and highest (exclusive). highest must be positive.int bounded(int highest)    //函数重载{Q_ASSERT(highest > 0);return int(bounded(0U, quint32(highest)));}qint64 bounded(qint64  highest) //形参的无符号与有符号版本{Q_ASSERT(highest > 0);return qint64(bounded(quint64(0), quint64(highest)));}quint64 bounded(quint64 highest);//Generates one random 32-bit quantity in the range between//lowest (inclusive) and highest (exclusive).//The highest parameter must be greater than lowest.quint32 bounded(quint32 lowest, quint32 highest){Q_ASSERT(highest > lowest);return bounded(highest - lowest) + lowest;}int bounded(int lowest, int highest){return bounded(highest - lowest) + lowest;}quint64 bounded(quint64 lowest, quint64 highest){Q_ASSERT(highest > lowest);return bounded(highest - lowest) + lowest;}qint64 bounded(qint64 lowest, qint64 highest){return bounded(highest - lowest) + lowest;}// these functions here only to help with ambiguous overloadsqint64 bounded(int lowest, qint64 highest){return bounded(qint64(lowest), qint64(highest));}qint64 bounded(qint64 lowest, int highest){return bounded(qint64(lowest), qint64(highest));}quint64 bounded(unsigned lowest, quint64 highest){return bounded(quint64(lowest), quint64(highest));}quint64 bounded(quint64 lowest, unsigned highest){return bounded(quint64(lowest), quint64(highest));}//Generates count 32- or 64-bit quantities (depending on the type UInt)//and stores them in the buffer pointed by buffer.//This is the most efficient way to obtain more than one quantity at a time,//as it reduces the number of calls into the Random Number Generator source.template <typename UInt, IfValidUInt<UInt> = true>void fillRange(UInt * buffer, qsizetype count){_fillRange(buffer, count * sizeof(UInt) / sizeof(quint32));}//Generates N 32- or 64-bit quantities (depending on the type UInt)//and stores them in the buffer array.template <typename UInt, size_t N, IfValidUInt<UInt> = true>void fillRange(UInt (&buffer)[N]) //形参是数组名的类型，N由模板参数确定了{_fillRange(buffer, N * sizeof(UInt) / sizeof(quint32));}// API like std:: random enginestypedef quint32 result_type; //重载了括号 () 运算符，使本类成为了可调用对象result_type operator()() { return generate(); }//Reseeds this object using the value seed as the seed.void seed(quint32 s = 1) { *this = { s }; }void seed(std::seed_seq & sseq) noexcept { *this = { sseq }; }//Discards the next z entries from the sequence.//This method is equivalent to calling generate() z times//and discarding the result, 形参是丢掉 z 个随机数Q_CORE_EXPORT void discard(unsigned long long z);//Returns the minimum value that QRandomGenerator may ever generate.//That is, 0.static constexpr result_type min(){ return std::numeric_limits<result_type>::min(); }//Returns the maximum value that QRandomGenerator may ever generate.//That is, std::numeric_limits<result_type>::max().static constexpr result_type max(){ return std::numeric_limits<result_type>::max(); }protected:enum System {}; //本类的 protected 属性的成员函数QRandomGenerator(System); //这是一个构造函数};class QRandomGenerator64 : public QRandomGenerator //子类的 64 bit 生成器
{QRandomGenerator64(System);
public:// unshadow generate() overloads, since we'll override.using QRandomGenerator::generate;quint64 generate() { return generate64(); }typedef quint64 result_type;result_type operator()() { return generate64(); }#ifndef Q_QDOCQRandomGenerator64(quint32 seedValue = 1): QRandomGenerator(seedValue) {}template <qsizetype N>QRandomGenerator64(const quint32 (&seedBuffer)[N]): QRandomGenerator(seedBuffer) {}QRandomGenerator64(const quint32 *seedBuffer, qsizetype len): QRandomGenerator(seedBuffer, len) {}QRandomGenerator64(std::seed_seq &sseq) noexcept: QRandomGenerator(sseq) {}QRandomGenerator64(const quint32 *begin, const quint32 *end): QRandomGenerator(begin, end) {}QRandomGenerator64(const QRandomGenerator &other): QRandomGenerator(other) {}void discard(unsigned long long z){Q_ASSERT_X(z * 2 > z, "QRandomGenerator64::discard","Overflow. Are you sure you want to skip over 9 quintillion samples?");QRandomGenerator::discard(z * 2);}static constexpr result_type min(){ return std::numeric_limits<result_type>::min(); }static constexpr result_type max(){ return std::numeric_limits<result_type>::max(); }static Q_DECL_CONST_FUNCTION Q_CORE_EXPORT QRandomGenerator64 *system();static Q_DECL_CONST_FUNCTION Q_CORE_EXPORT QRandomGenerator64 *global();static Q_CORE_EXPORT QRandomGenerator64 securelySeeded();
#endif // Q_QDOC
};inline quint64 QRandomGenerator::bounded(quint64 highest)
{// Implement an algorithm similar to libc++'s uniform_int_distribution:// loop around getting a random number, mask off any bits that "highest"// will never need, then check if it's higher than "highest". The number of// times the loop will run is unbounded but the probability of terminating// is better than 1/2 on each iteration. Therefore, the average loop count// should be less than 2.const int width = qCountLeadingZeroBits(highest - 1);const quint64 mask = (quint64(1) << (std::numeric_limits<quint64>::digits - width)) - 1;quint64 v;do {v = generate64() & mask;} while (v >= highest);return v;
}inline QRandomGenerator * QRandomGenerator::system()
{   //本函利用系统资源生成随机数生成器return QRandomGenerator64::system();
}inline QRandomGenerator * QRandomGenerator::global()
{return QRandomGenerator64::global();
}//由这里可见，子类的 64 bit的随机数生成器更重要，更基础。QRandomGenerator QRandomGenerator::securelySeeded()
{return QRandomGenerator64::securelySeeded();
}QT_END_NAMESPACE#endif // QRANDOM_H

（2）

谢谢