How to Create a Random String in C++

Generating a random string in C++ is a simple task that can be easily achieved by the use of rand(); however, this method is not recommended because rand() lacks a distribution engine and the quality of its implementation leaves more to be desired. To combat this issue, the C++ Standard Library provides a set of functions from the Random header that allows users to generate random numbers from well-defined engines and distributions. The implementation of the function that generates a random string will make use of three key features present in the facilities of the Random header.

The elements used being:

The implementation for the random string generator combining all these features together is shown below:

std::string random_string(std::size_t length)
    const std::string characters = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";

    std::random_device random_device;
    std::mt19937 generator(random_device());
    std::uniform_int_distribution<> distribution(0, characters.size() - 1);

    std::string random_string;

    for (std::size_t i = 0; i < length; ++i)
        random_string += characters[distribution(generator)];

    return random_string;

As you can see, this function hard codes all the possible characters that will be used to form the random string. Then, it initializes the three needed objects from the Random header by creating a Mersenne Twister engine and a distribution that covers all indexes from the possible characters string. Ultimately, it iterates through the arbitrary length given as a parameter to populate the string with a random character in every iteration.


Avoiding the use of rand() is a piece of advice that does not only apply to create random strings. Instead, it is recommended to use the facilities provided in the Random header that were included in the C++11 Standard Library.

Source Code