C++ How to Program, 10/e

... identifiers that begin with underscores and double underscores, because C++ compilers use names like that for their own purposes internally.

Placement of Variable Declarations

Declarations of variables can be placed almost anywhere in a program, but they must appear before their corresponding variables are used in the program. For example, in the program of Fig. 2.5, the declaration in line 8

int number1{0}; // first integer to add (initialized to 0)

could have been placed immediately before line 13

std::cin >> number1; // read first integer from user into number1

the declaration in line 9

int number2{0}; // second integer to add (initialized to 0)

could have been placed immediately before line 16

std::cin >> number2; // read second integer ...