I am new to Visual Studio. I just followed a tutorial and successfully wrote and ran a simple program. But after I closed Visual Studio and double click the cpp file I just wrote. I can't find the build menu in the Visual Studio. Could anyone tell me how to open and run the existing cpp file? I need to do some debug and don't want to begin a new project and copy the code again and again. Thanks!
You need to open more than the .cpp file alone.
Make sure you open the whole project or solution (File/Open/Project - .sln, .vcxproj, .vcproj). Double clicking on a .cpp file will open that file only.
Then you should be able to build the project (or hit Ctrl-F5).