VSCode - C++ undefined reference to 'CLASS::FUNCTION'

I'm using VSCode to create a C++ project and keep getting this error when trying to build && debug. When running from console if I use 'g++ -o main 'main.cpp' 'file_1.cpp' 'file_2.cpp'' it works and compiles correctly.

I have read that this is something to do with the linking of files? Does anyone know how to fix this in VSCode? I have a default launch.json configuration file that builds the active file if this helps.

Here is the contents of my tasks.json file:

{
"version": "2.0.0",
"tasks": [
    {
        "type": "shell",
        "label": "shell: g++ build active file",
        "command": "/usr/bin/g++",
        "args": [
            "-g",
            "${file}",
            "-o",
            "${fileDirname}/${fileBasenameNoExtension}"
        ],
        "options": {
            "cwd": "/usr/bin"
        },
        "problemMatcher": [
            "$gcc"
        ],
        "group": {
            "kind": "build",
            "isDefault": true
        }
    },
    {
        "type": "shell",
        "label": "g++ build active file",
        "command": "/usr/bin/g++",
        "args": [
            "-g",
            "${file}",
            "-o",
            "${fileDirname}/${fileBasenameNoExtension}"
        ],
        "options": {
            "cwd": "/usr/bin"
        }
    }
]

}

EDIT: I'm using Debian 10 'Buster'

Many thanks