Error c3861: identifier not found | identifier not found

Just began to learn C++, today encountered more depressed problems, maybe beginners will encounter this problem, take it out to share.
Define a function, and then call in the main function, compile a run, prompt me “: identifier cannot be found “… , using VS2010.
I looked at it so many times, couldn’t find the solution, and finally found the answer on StackOverflow,
The compiler compiles the contents of the file from top to bottom, and the function A is called in the main function, but because the main function is defined before the function A, the function A is not defined when it is used, so the “identifier cannot be found” situation occurs.
So, you just have to move the function forward.

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