What Are The Differences Between std::rand() And std::mt19937 In Modern C++?

Random numbers are widely used in today’s modern applications. In C we use rand(), srand() and in C++ it is std::rand(), std::srand(). Since C++11, we can use the Mersenne Twister random generators;  mt19937 (std::mt19937) for 32-bit applications and mt19937_64 (std::mt19937_64) for 64-bit applications. Modern C++ allows us to use both old and new random generators. In this post, we explain what are the differences between rand(), std::rand(), std::mt19937(), std::mt19937_64().

What is a random number and rand() in C and C++?

A random number is a number that is randomly chosen in a given range. It is statistically impossible to predict future values based on past or present values and they are uniformly distributed over a defined interval or set.

The rand() is a C function defined in <stdlib.h> and std::rand() is a C++ function defined in the <cstdlib> header. Both are the same and are used to generate pseudo-random numbers in the range of 0 to RAND_MAX. Here is the definition in the <cstdlib> header.

In C we use rand(), srand() and in C++ we use std::rand(), std::srand() they use a Linear Congruential Generator. While they are added to <cstdlib> to make modern C++ compatible, there a more modern random number generator algorithms, such as Mersenne Twister algorithm.

How can we use rand() in C?

Here is a simple C example to generate a random number between 0 to 100,

How can we use std::rand() in C++?

Here is a simple C++ example to generate a random number between 0 to 100,

What is Mersenne Twister Algorithm in programming?

A Mersenne prime is a prime number used in mathematics that is a number of the form Mn = 2n − 1 where the n is an integer. The Mersenne Twister is a pseudorandom number generator where the period length is chosen to be a Mersenne Prime. It was developed by Makoto Matsumoto in 1997.

Since C++11, the Mersenne Twister mathematical number generator is implemented as a random generator number, it is defined in <random> header as a std::mersenne_twister_engine that is a random number engine based on Mersenne Twister algorithm.

What is std::mt19937 random number generator in modern C++?

In modern C++, there are more useful and modern random number generators. These are std::mt19937 and std::mt19937_64. The std::mt19937 is a 32-bit Mersenne Twister by Matsumoto and Nishimura in 1998, and std::mt19937_64 is a 64-bit Mersenne Twister by Matsumoto and Nishimura in 2000.

The std::mt19937 is a random number generator defined in <random> header in C++17 standard and beyond, producing 32-bit pseudo-random numbers by using the Mersenne Twister algorithm with a state size of 19937 bits. This is why it is called mt19937 and there is a 64-bit version called mt19937_64. Both are defined as an instantiation of the mersenne_twister_engine. Now let’s see their definitions.

Since C++11, mt19937 is defined as below,

Since C++11, mt19937_64 is defined as below,

Is there a simple example to use std::mt19937 in modern C++?

Here is a simple example to use std::mt19937,

What are the differences between std::rand() and std::mt19937 in modern C++?

As you see we define both rand and mt19937 above. Now let’s list their differences between them.

	rand	std::rand	std::mt19937	std::mt19937_64
Generator Type	Linear Congruential	Linear Congruential	Mersenne Twister	Mersenne Twister
Defined Header	stdlib.h	cstdlib	random	random
Used In	C and C++	C++	C++11	C++11
Bits	16-bit	16-bit or 32-bit	32-bit	64-bit
Minimum Number	0	0	0	0
Maximum Number	32767	32767 or 2147483647	4123659995	9981545732273789042
Min, Max Number Definition	0 RAND_MAX	0 RAND_MAX	.min() .max()	.min() .max()
Seeding New Generator	srand()	std::srand()	.seed()	.seed()
Modern or Classic	classic C	modern C++	modern C++	modern C++
Return type	int	int	uint_fast32_t	uint_fast64_t
Recommended in C++?	No	No	Yes	Yes
Random Period	Shorter	Shorter	Longer	Much longer
Randomness	Low	Low	High	High
Quality of the Random Sequence	No guarantee	No guarantee	Good	Good
Using with Iterators and Containers	No	Hard to use and Slow	Easy and Fast	Easy and Useful in 64bit
Safety	Not safe	Not safe	Safe	Safe
Coding	Easy	hard to remember it’s header name	hard to remember its name cuz of 19937	hard to remember its name cuz of 19937_64

For more details about this feature in C++11 standard, please see these papers; p0205r1

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