Difference between revisions of "cpp/numeric/random/random device"
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{{cpp/title|random_device}} | {{cpp/title|random_device}} | ||
{{cpp/numeric/random/random_device/navbar}} | {{cpp/numeric/random/random_device/navbar}} | ||
| − | {{ddcl | header=random | since=c++11 | 1= | + | {{ddcl|header=random|since=c++11|1= |
class random_device; | class random_device; | ||
}} | }} | ||
| Line 7: | Line 7: | ||
{{tt|std::random_device}} is a uniformly-distributed integer random number generator that produces non-deterministic random numbers. | {{tt|std::random_device}} is a uniformly-distributed integer random number generator that produces non-deterministic random numbers. | ||
| − | {{tt|std::random_device}} may be implemented in terms of an implementation-defined pseudo-random number engine if a non-deterministic source (e.g. a hardware device) is not available to the implementation. In this case each {{ | + | {{tt|std::random_device}} may be implemented in terms of an implementation-defined pseudo-random number engine if a non-deterministic source (e.g. a hardware device) is not available to the implementation. In this case each {{ttt|std::random_device}} object may generate the same number sequence. |
===Member types=== | ===Member types=== | ||
{{dsc begin}} | {{dsc begin}} | ||
| − | {{dsc hitem | Member type | Definition}} | + | {{dsc hitem|Member type|Definition}} |
| − | {{dsc | {{tt|result_type}} | + | {{dsc|{{tt|result_type}} {{mark c++11}}|{{co|unsigned int}}}} |
{{dsc end}} | {{dsc end}} | ||
===Member functions=== | ===Member functions=== | ||
{{dsc begin}} | {{dsc begin}} | ||
| − | {{dsc h2 | Construction}} | + | {{dsc h2|Construction}} |
| − | {{dsc inc | cpp/numeric/random/engine/dsc constructor |random_device}} | + | {{dsc inc|cpp/numeric/random/engine/dsc constructor|random_device}} |
| − | {{dsc mem fun | nolink=true | operator{{=}} | notes={{mark|deleted}} | the assignment operator is deleted}} | + | {{dsc mem fun|nolink=true|operator{{=}}|notes={{mark|deleted}} {{mark c++11}}|the assignment operator is deleted}} |
| − | {{dsc h2 | Generation}} | + | {{dsc h2|Generation}} |
| − | {{dsc inc | cpp/numeric/random/engine/dsc operator() | random_device}} | + | {{dsc inc|cpp/numeric/random/engine/dsc operator()|random_device}} |
| − | {{dsc h2 | Characteristics}} | + | {{dsc h2|Characteristics}} |
| − | {{dsc inc | cpp/numeric/random/random_device/dsc entropy}} | + | {{dsc inc|cpp/numeric/random/random_device/dsc entropy}} |
| − | {{dsc inc | cpp/numeric/random/engine/dsc min |random_device}} | + | {{dsc inc|cpp/numeric/random/engine/dsc min|random_device}} |
| − | {{dsc inc | cpp/numeric/random/engine/dsc max |random_device}} | + | {{dsc inc|cpp/numeric/random/engine/dsc max|random_device}} |
{{dsc end}} | {{dsc end}} | ||
===Notes=== | ===Notes=== | ||
| − | A notable implementation where {{ | + | A notable implementation where {{ttt|std::random_device}} is deterministic in old versions of MinGW-w64 ([https://sourceforge.net/p/mingw-w64/bugs/338/ bug 338], fixed since GCC 9.2). The latest MinGW-w64 versions can be downloaded from [https://gcc-mcf.lhmouse.com/ GCC with the MCF thread model]. |
===Example=== | ===Example=== | ||
{{example | {{example | ||
| − | + | |code= | |
| − | + | ||
#include <iostream> | #include <iostream> | ||
| − | |||
#include <map> | #include <map> | ||
#include <random> | #include <random> | ||
| + | #include <string> | ||
int main() | int main() | ||
| Line 47: | Line 46: | ||
std::map<int, int> hist; | std::map<int, int> hist; | ||
std::uniform_int_distribution<int> dist(0, 9); | std::uniform_int_distribution<int> dist(0, 9); | ||
| − | for (int n = 0; n | + | |
| − | ++hist[dist(rd)]; // note: demo only: the performance of many | + | for (int n = 0; n != 20000; ++n) |
| + | ++hist[dist(rd)]; // note: demo only: the performance of many | ||
// implementations of random_device degrades sharply | // implementations of random_device degrades sharply | ||
// once the entropy pool is exhausted. For practical use | // once the entropy pool is exhausted. For practical use | ||
| − | // random_device is generally only used to seed | + | // random_device is generally only used to seed |
// a PRNG such as mt19937 | // a PRNG such as mt19937 | ||
| − | + | ||
| − | for (auto | + | for (auto [x, y] : hist) |
| − | std::cout << | + | std::cout << x << " : " << std::string(y / 100, '*') << '\n'; |
| − | + | ||
} | } | ||
| − | + | |p=true | |
| + | |output= | ||
0 : ******************** | 0 : ******************** | ||
1 : ******************* | 1 : ******************* | ||
Latest revision as of 21:34, 20 November 2023
| Defined in header <random>
|
||
| class random_device; |
(since C++11) | |
std::random_device is a uniformly-distributed integer random number generator that produces non-deterministic random numbers.
std::random_device may be implemented in terms of an implementation-defined pseudo-random number engine if a non-deterministic source (e.g. a hardware device) is not available to the implementation. In this case each std::random_device object may generate the same number sequence.
Contents |
[edit] Member types
| Member type | Definition |
result_type (C++11)
|
unsigned int |
[edit] Member functions
Construction | |
| constructs the engine (public member function) | |
| operator= (deleted) (C++11) |
the assignment operator is deleted (public member function) |
Generation | |
| advances the engine's state and returns the generated value (public member function) | |
Characteristics | |
| (C++11) |
obtains the entropy estimate for the non-deterministic random number generator (public member function) |
| [static] |
gets the smallest possible value in the output range (public static member function) |
| [static] |
gets the largest possible value in the output range (public static member function) |
[edit] Notes
A notable implementation where std::random_device is deterministic in old versions of MinGW-w64 (bug 338, fixed since GCC 9.2). The latest MinGW-w64 versions can be downloaded from GCC with the MCF thread model.
[edit] Example
Run this code
#include <iostream> #include <map> #include <random> #include <string> int main() { std::random_device rd; std::map<int, int> hist; std::uniform_int_distribution<int> dist(0, 9); for (int n = 0; n != 20000; ++n) ++hist[dist(rd)]; // note: demo only: the performance of many // implementations of random_device degrades sharply // once the entropy pool is exhausted. For practical use // random_device is generally only used to seed // a PRNG such as mt19937 for (auto [x, y] : hist) std::cout << x << " : " << std::string(y / 100, '*') << '\n'; }
Possible output:
0 : ******************** 1 : ******************* 2 : ******************** 3 : ******************** 4 : ******************** 5 : ******************* 6 : ******************** 7 : ******************** 8 : ******************* 9 : ********************
