You can now create declarations from definitions and vice versa in Visual Studio Code.

VS Code C++ Extension January Update: Create Definitions and Declarations

by Alexandra Kemper

From the article:

Starting with the 1.13.6 version of the C++ Extension in VS Code, we are happy to share a much requested feature: Auto creation of definitions or declarations for functions! You can now quickly create a declaration in a header file for a function you only have a definition for, or vice versa. These generated definitions and declarations already include all function arguments, so no need for constant copying, pasting, and double checking.