#include <iostream>
#include <vector>
#include <random>
#include <functional> //for std::function
#include <algorithm>  //for std::generate_n
 
typedef std::vector<char> char_array;
 
//we don't want a global. That's ugly.
//This will get optimized at compile time anyway
//http://e...content-available-to-author-only...a.org/wiki/Return_value_optimization
char_array charset()
{
    //Change this to suit
    return char_array( 
	{'0','1','2','3','4',
	'5','6','7','8','9',
	'A','B','C','D','E','F',
	'G','H','I','J','K',
	'L','M','N','O','P',
	'Q','R','S','T','U',
	'V','W','X','Y','Z',
	'a','b','c','d','e','f',
	'g','h','i','j','k',
	'l','m','n','o','p',
	'q','r','s','t','u',
	'v','w','x','y','z'
	});
};    
 
std::string random_string( size_t length, std::function<char(void)> rand_char )
{
    std::string str(length,0);
    std::generate_n( str.begin(), length, rand_char );
    return str;
}
 
int main()
{
    //Independent of character set, 
    //Distribution can be changed
    //Output is non-deterministic
    const auto ch_set = charset();
    std::default_random_engine rng(std::random_device{}()); 
    std::uniform_int_distribution<> dist(0, ch_set.size()-1);
    auto randchar = [ ch_set,&dist,&rng ](){return ch_set[ dist(rng) ];};
    auto length = 5;
    std::cout<<random_string(length,randchar)<<std::endl;
    return 0;
}

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