Waits for input for certain time frame

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C++

I am just a beginner of C++ and trying to get a simple function done.

Lets say i have these first 3 lines:

int age;
cout<<"please enter your age"<<endl;
cin>>age;


I want this function to wait for 30 seconds, if there is any input of integer, function A is executed, else there is no input by the user and the time is up, function B is executed.

I know the function sleep works well for waiting, but the program waits until there is input, not stopping after 30 seconds.

Please help, thank you.