QRandomGenerator Class▲

Header: QRandomGenerator

Since: Qt 5.10

CMake:
find_package(Qt6 REQUIRED COMPONENTS Core)
target_link_libraries(mytarget PRIVATE Qt6::Core)

qmake: QT += core

Inherited By: QRandomGenerator64
Detailed Description▲
QRandomGenerator may be used to generate random values from a highquality random number generator. Like the C++ random engines, QRandomGenerator can be seeded with userprovided values through the constructor. When seeded, the sequence of numbers generated by this class is deterministic. That is to say, given the same seed data, QRandomGenerator will generate the same sequence of numbers. But given different seeds, the results should be considerably different.
QRandomGenerator::securelySeeded() can be used to create a QRandomGenerator that is securely seeded with QRandomGenerator::system(), meaning that the sequence of numbers it generates cannot be easily predicted. Additionally, QRandomGenerator::global() returns a global instance of QRandomGenerator that Qt will ensure to be securely seeded. This object is threadsafe, may be shared for most uses, and is always seeded from QRandomGenerator::system()
QRandomGenerator::system() may be used to access the system's cryptographicallysafe random generator. On Unix systems, it's equivalent to reading from /dev/urandom or the getrandom() or getentropy() system calls.
The class can generate 32bit or 64bit quantities, or fill an array of those. The most common way of generating new values is to call the generate(), generate64() or fillRange() functions. One would use it as:
quint32 value =
QRandomGenerator::
global()&
gt;generate();
Additionally, it provides a floatingpoint function generateDouble() that returns a number in the range [0, 1) (that is, inclusive of zero and exclusive of 1). There's also a set of convenience functions that facilitate obtaining a random number in a bounded, integral range.
Seeding and determinism▲
QRandomGenerator may be seeded with specific seed data. When that is done, the numbers generated by the object will always be the same, as in the following example:
QRandomGenerator prng1(1234
), prng2(1234
);
Q_ASSERT(prng1.generate() ==
prng2.generate());
Q_ASSERT(prng1.generate64() ==
prng2.generate64());
The seed data takes the form of one or more 32bit words. The ideal seed size is approximately equal to the size of the QRandomGenerator class itself. Due to mixing of the seed data, QRandomGenerator cannot guarantee that distinct seeds will produce different sequences.
QRandomGenerator::global(), like all generators created by QRandomGenerator::securelySeeded(), is always seeded from QRandomGenerator::system(), so it's not possible to make it produce identical sequences.
Bulk data▲
When operating in deterministic mode, QRandomGenerator may be used for bulk data generation. In fact, applications that do not need cryptographicallysecure or true random data are advised to use a regular QRandomGenerator instead of QRandomGenerator::system() for their random data needs.
For ease of use, QRandomGenerator provides a global object that can be easily used, as in the following example:
int
x =
QRandomGenerator::
global()&
gt;generate();
int
y =
QRandomGenerator::
global()&
gt;generate();
int
w =
QRandomGenerator::
global()&
gt;bounded(16384
);
int
h =
QRandomGenerator::
global()&
gt;bounded(16384
);
Systemwide random number generator▲
QRandomGenerator::system() may be used to access the systemwide random number generator, which is cryptographicallysafe on all systems that Qt runs on. This function will use hardware facilities to generate random numbers where available. On such systems, those facilities are true Random Number Generators. However, if they are true RNGs, those facilities have finite entropy sources and thus may fail to produce any results if their entropy pool is exhausted.
If that happens, first the operating system then QRandomGenerator will fall back to Pseudo Random Number Generators of decreasing qualities (Qt's fallback generator being the simplest). Whether those generators are still of cryptographic quality is implementationdefined. Therefore, QRandomGenerator::system() should not be used for highfrequency random number generation, lest the entropy pool become empty. As a rule of thumb, this class should not be called upon to generate more than a kilobyte per second of random data (note: this may vary from system to system).
If an application needs true RNG data in bulk, it should use the operating system facilities (such as /dev/random on Linux) directly and wait for entropy to become available. If the application requires PRNG engines of cryptographic quality but not of true randomness, QRandomGenerator::system() may still be used (see section below).
If neither a true RNG nor a cryptographically secure PRNG are required, applications should instead use PRNG engines like QRandomGenerator's deterministic mode and those from the C++ Standard Library. QRandomGenerator::system() can be used to seed those.
Fallback quality▲
QRandomGenerator::system() uses the operating system facilities to obtain random numbers, which attempt to collect real entropy from the surrounding environment to produce true random numbers. However, it's possible that the entropy pool becomes exhausted, in which case the operating system will fall back to a pseudorandom engine for a time. Under no circumstances will QRandomGenerator::system() block, waiting for more entropy to be collected.
The following operating systems guarantee that the results from their randomgeneration API will be of at least cryptographicallysafe quality, even if the entropy pool is exhausted: Apple OSes (Darwin), BSDs, Linux, Windows. Barring a system installation problem (such as /dev/urandom not being readable by the current process), QRandomGenerator::system() will therefore have the same guarantees.
On other operating systems, QRandomGenerator will fall back to a PRNG of good numeric distribution, but it cannot guarantee proper seeding in all cases. Please consult the OS documentation for more information.
Applications that require QRandomGenerator not to fall back to noncryptographic quality generators are advised to check their operating system documentation or restrict their deployment to one of the above.
Reentrancy and threadsafety▲
QRandomGenerator is reentrant, meaning that multiple threads can operate on this class at the same time, so long as they operate on different objects. If multiple threads need to share one PRNG sequence, external locking by a mutex is required.
The exceptions are the objects returned by QRandomGenerator::global() and QRandomGenerator::system(): those objects are threadsafe and may be used by any thread without external locking. Note that threadsafety does not extend to copying those objects: they should always be used by reference.
Standard C++ Library compatibility▲
QRandomGenerator is modeled after the requirements for random number engines in the C++ Standard Library and may be used in almost all contexts that the Standard Library engines can. Exceptions to the requirements are the following:

QRandomGenerator does not support seeding from another seed sequencelike class besides std::seed_seq itself;

QRandomGenerator is not comparable (but is copyable) or streamable to std::ostream or from std::istream.
QRandomGenerator is also compatible with the uniform distribution classes std::uniform_int_distribution and std:uniform_real_distribution, as well as the free function std::generate_canonical. For example, the following code may be used to generate a floatingpoint number in the range [1, 2.5):
std::
uniform_real_distribution dist(1
, 2.5
);
return
dist(*
QRandomGenerator::
global());
See Also▲
See also QRandomGenerator64
Member Type Documentation▲
QRandomGenerator::result_type▲
Member Function Documentation▲
qint64 QRandomGenerator::bounded(int lowest, qint64 highest)▲
qint64 QRandomGenerator::bounded(qint64 lowest, int highest)
quint64 QRandomGenerator::bounded(quint64 lowest, unsigned int highest)
quint64 QRandomGenerator::bounded(unsigned int lowest, quint64 highest)
This is an overloaded function.
This function exists to help with overload resolution when the types of the parameters don't exactly match. They will promote the smaller type to the type of the larger one and call the correct overload.
QRandomGenerator::QRandomGenerator(quint32 seedValue = 1)▲
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.
See Also▲
See also seed(), securelySeeded()
QRandomGenerator::QRandomGenerator(const quint32 (&)[N] seedBuffer = N)▲
This is an overloaded function.
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.
See Also▲
See also seed(), securelySeeded()
QRandomGenerator::QRandomGenerator(const quint32 *seedBuffer, qsizetype len)▲
This is an overloaded function.
Initializes this QRandomGenerator object with len 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.
This constructor is equivalent to:
std::
seed_seq sseq(seedBuffer, seedBuffer +
len);
QRandomGenerator generator(sseq);
See Also▲
See also seed(), securelySeeded()
QRandomGenerator::QRandomGenerator(std::seed_seq &sseq)▲
This is an overloaded function.
Initializes this QRandomGenerator object with the seed sequence sseq as the seed. Two objects constructed or reseeded with the same seed value will produce the same number sequence.
See Also▲
See also seed(), securelySeeded()
QRandomGenerator::QRandomGenerator(const quint32 *begin, const quint32 *end)▲
This is an overloaded function.
Initializes this QRandomGenerator object with the values found in the range from begin to end as the seed. Two objects constructed or reseeded with the same seed value will produce the same number sequence.
This constructor is equivalent to:
std::
seed_seq sseq(begin, end);
QRandomGenerator generator(sseq);
See Also▲
See also seed(), securelySeeded()
QRandomGenerator::QRandomGenerator(const QRandomGenerator &other)▲
Creates a copy of the generator state in the other object. If other is QRandomGenerator::system() or a copy of that, this object will also read from the operating system randomgenerating facilities. In that case, the sequences generated by the two objects will be different.
In all other cases, the new QRandomGenerator object will start at the same position in the deterministic sequence as the other object was. Both objects will generate the same sequence from this point on.
For that reason, it is not advisable to create a copy of QRandomGenerator::global(). If one needs an exclusive deterministic generator, consider instead using securelySeeded() to obtain a new object that shares no relationship with the QRandomGenerator::global().
double QRandomGenerator::bounded(double highest)▲
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;
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).
See Also▲
See also generateDouble(), bounded()
quint32 QRandomGenerator::bounded(quint32 highest)▲
This is an overloaded function.
Generates one random 32bit quantity in the range between 0 (inclusive) and highest (exclusive). The same result may also be obtained by using std::uniform_int_distribution with parameters 0 and highest  1. That class can also be used to obtain quantities larger than 32 bits; for 64 bits, the 64bit bounded() overload can be used too.
For example, to obtain a value between 0 and 255 (inclusive), one would write:
quint32 v =
QRandomGenerator::
global()&
gt;bounded(256
);
Naturally, the same could also be obtained by masking the result of generate() to only the lower 8 bits. Either solution is as efficient.
Note that this function cannot be used to obtain values in the full 32bit range of quint32. Instead, use generate().
See Also▲
See also generate(), generate64(), generateDouble()
quint32 QRandomGenerator::bounded(quint32 lowest, quint32 highest)▲
This is an overloaded function.
Generates one random 32bit quantity in the range between lowest (inclusive) and highest (exclusive). The highest parameter must be greater than lowest.
The same result may also be obtained by using std::uniform_int_distribution with parameters lowest and \a highest  1. That class can also be used to obtain quantities larger than 32 bits.
For example, to obtain a value between 1000 (incl.) and 2000 (excl.), one would write:
quint32 v =
QRandomGenerator::
global()&
gt;bounded(1000
, 2000
);
Note that this function cannot be used to obtain values in the full 32bit range of quint32. Instead, use generate().
See Also▲
See also generate(), generate64(), generateDouble()
int QRandomGenerator::bounded(int highest)▲
This is an overloaded function.
Generates one random 32bit quantity in the range between 0 (inclusive) and highest (exclusive). highest must be positive.
Note that this function cannot be used to obtain values in the full 32bit range of int. Instead, use generate() and cast to int.
See Also▲
See also generate(), generate64(), generateDouble()
int QRandomGenerator::bounded(int lowest, int highest)▲
This is an overloaded function.
Generates one random 32bit quantity in the range between lowest (inclusive) and highest (exclusive), both of which may be negative, but highest must be greater than lowest.
Note that this function cannot be used to obtain values in the full 32bit range of int. Instead, use generate() and cast to int.
See Also▲
See also generate(), generate64(), generateDouble()
quint64 QRandomGenerator::bounded(quint64 highest)▲
This is an overloaded function.
Generates one random 64bit quantity in the range between 0 (inclusive) and highest (exclusive). The same result may also be obtained by using std::uniform_int_distribution<quint64> with parameters 0 and highest  1.
Note that this function cannot be used to obtain values in the full 64bit range of quint64. Instead, use generate64().
This function is implemented as a loop, which depends on the random value obtained. On the long run, on average it should loop just under 2 times, but if the random generator is defective, this function may take considerably longer to execute.
See Also▲
See also generate(), generate64(), generateDouble()
quint64 QRandomGenerator::bounded(quint64 lowest, quint64 highest)▲
This is an overloaded function.
Generates one random 64bit quantity in the range between lowest (inclusive) and highest (exclusive). The highest parameter must be greater than lowest.
The same result may also be obtained by using std::uniform_int_distribution<quint64> with parameters lowest and \a highest  1.
Note that this function cannot be used to obtain values in the full 64bit range of quint64. Instead, use generate64().
This function is implemented as a loop, which depends on the random value obtained. On the long run, on average it should loop just under 2 times, but if the random generator is defective, this function may take considerably longer to execute.
See Also▲
See also generate(), generate64(), generateDouble()
qint64 QRandomGenerator::bounded(qint64 highest)▲
This is an overloaded function.
Generates one random 64bit quantity in the range between 0 (inclusive) and highest (exclusive). highest must be positive.
Note that this function cannot be used to obtain values in the full 64bit range of qint64. Instead, use generate64() and cast to qint64 or instead use the unsigned version of this function.
This function is implemented as a loop, which depends on the random value obtained. On the long run, on average it should loop just under 2 times, but if the random generator is defective, this function may take considerably longer to execute.
See Also▲
See also generate(), generate64(), generateDouble()
qint64 QRandomGenerator::bounded(qint64 lowest, qint64 highest)▲
This is an overloaded function.
Generates one random 64bit quantity in the range between lowest (inclusive) and highest (exclusive), both of which may be negative, but highest must be greater than lowest.
Note that this function cannot be used to obtain values in the full 64bit range of qint64. Instead, use generate64() and cast to qint64.
This function is implemented as a loop, which depends on the random value obtained. On the long run, on average it should loop just under 2 times, but if the random generator is defective, this function may take considerably longer to execute.
See Also▲
See also generate(), generate64(), generateDouble()
void QRandomGenerator::discard(unsigned long long z)▲
Discards the next z entries from the sequence. This method is equivalent to calling generate() z times and discarding the result, as in:
while
(z
)
generator.generate();
void QRandomGenerator::fillRange(UInt *buffer, qsizetype count)▲
Generates count 32 or 64bit 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.
For example, to fill a list of 16 entries with random values, one may write:
QList&
lt;quint32&
gt; list;
list.resize(16
);
QRandomGenerator::
global()&
gt;fillRange(list.data(), list.size());
See Also▲
See also generate()
void QRandomGenerator::fillRange(UInt (&)[N] buffer = N)▲
Generates N 32 or 64bit quantities (depending on the type UInt) and stores them in the buffer array. 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.
For example, to fill generate two 32bit quantities, one may write:
quint32 array[2
];
QRandomGenerator::
global()&
gt;fillRange(array);
It would have also been possible to make one call to generate64() and then split the two halves of the 64bit value.
See Also▲
See also generate()
quint64 QRandomGenerator::generate64()▲
quint32 QRandomGenerator::generate()▲
void QRandomGenerator::generate(ForwardIterator begin, ForwardIterator end)▲
Generates 32bit quantities and stores them in the range between begin and end. This function is equivalent to (and is implemented as):
std::
generate(begin, end, [this
]() {
return
generate(); }
);
This function complies with the requirements for the function std::seed_seq::generate, which requires unsigned 32bit integer values.
Note that if the [begin, end) range refers to an area that can store more than 32 bits per element, the elements will still be initialized with only 32 bits of data. Any other bits will be zero. To fill the range with 64 bit quantities, one can write:
std::
generate(begin, end, []() {
return
QRandomGenerator::
global()&
gt;generate64(); }
);
If the range refers to contiguous memory (such as an array or the data from a QList), the fillRange() function may be used too.
See Also▲
See also fillRange()
double QRandomGenerator::generateDouble()▲
Generates one random qreal in the canonical range [0, 1) (that is, inclusive of zero and exclusive of 1).
This function is equivalent to:
QRandomGenerator64 rd;
return
std::
generate_canonical&
lt;qreal, std::
numeric_limits&
lt;qreal&
gt;::
digits&
gt;(rd);
The same may also be obtained by using std::uniform_real_distribution with parameters 0 and 1.
See Also▲
See also generate(), generate64(), bounded()
[static] QRandomGenerator *QRandomGenerator::global()▲
Returns a pointer to a shared QRandomGenerator that was seeded using securelySeeded(). This function should be used to create random data without the expensive creation of a securelyseeded QRandomGenerator for a specific use or storing the rather large