How can i genrate a random number sampled from exponention disribution in c++?

Using C++, i want to generate a random number representing a period of time with a mean duration of 40 sec sampled from an exponential distribution. When i checked this site http://www.cplusplus.com/reference/random/exponential_distribution/ about this distribution, i found that the only parameter in this distribution is lambda(λ) nothing about mean in this distribution. i don't know if there is any relationship between lambda and mean. Please, any help or explanation?