Method 1 
--------

If you use Code Runner:
- Go Extension
        -> Click "Code Runner" 's gear icon
        -> Click "Configure Extension Settings"
        -> Search "Code-runner: Executor Map"
        -> Click "Edit in settings.json"
        -> Search for "cpp:" under "code-runner.executorMap"
        -> Add the flag "-std=c++11" after "g++"

Or you may edit the file "settings.json" directly using an editor.  On my Mac, the file is located as:
        ~/Library/Application\ Support/Code/User/settings.json


Method 2 
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If you don't use Code Runner, configure the building process by:

- Go Terminal
        -> Configure Default Build Task
        -> add the compilation flag

- To compile,
        -> Terminal 
        -> Run Build Task