Defined in header <random>  

template< class RealType = double > class gamma_distribution;  (since C++11) 
Produces random positive floatingpoint values x, distributed according to probability density function: P(xα,β) =
ex/β 
βα · Γ(α) 
where α is known as the shape parameter and β is known as the scale parameter. The shape parameter is sometimes denoted by the letter k and the scale parameter is sometimes denoted by the letter θ.
For floatingpoint α, the value obtained is the sum of α independent exponentially distributed random variables, each of which has a mean of β
std::gamma_distribution
satisfies RandomNumberDistribution
.
RealType    The result type generated by the generator. The effect is undefined if this is not one of float , double , or long double . 
Member type  Definition 

result_type  RealType 
param_type  the type of the parameter set, see RandomNumberDistribution . 
constructs new distribution (public member function) 

resets the internal state of the distribution (public member function) 

Generation 

generates the next random number in the distribution (public member function) 

Characteristics 

returns the distribution parameters (public member function) 

gets or sets the distribution parameter object (public member function) 

returns the minimum potentially generated value (public member function) 

returns the maximum potentially generated value (public member function) 
compares two distribution objects (function) 

performs stream input and output on pseudorandom number distribution (function template) 
Weisstein, Eric W. "Gamma Distribution." From MathWorldA Wolfram Web Resource.
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