Hi,
there are some conventions in C++ language in naming the variables, objects, functions, etc... . And for example the convention for naming classes are: the 1st letter should be UPPERCASE and there shouldn't be any "_"-underscore character to divide letters but there should be each new word written with UPPERCASE letter: For example: "SimpleClass" or "MyNewClass".
Or one example for writing functions: "simpleFunction()", "myFunction()".
AND THE QUESTION IS:
Why for example the class from <string> library is written as "string" (or "basic_string<char>") or some variables are written as "out_of_scope". So that they are written WITH UNDERSCORE and classes WITH SMALL LETTERS?
No - there no 'standards' for naming variables, etc in the way you described.
Several organisations have their own self styled conventions and they are all different.
You can do your own so long as you use it consistently.