How to make an alphanumeric with restrictions

I want to make an alphanumeric string generator that uses capital letters and numbers to make a random string. I also want it to only use 7 numbers and 18 letters and mix them randomly together. I'm not sure on how to do this! please help.
1) pick 18 letters, put them in an array.
2) pick 7 numbers, put them in the same array.
3) Use std::random_shuffle to randomize the array.

Reference with example:
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