How to get known paths for linux

Windows has a concept of a Known Path with functions to retrieve them without hard-coding a path:

#include <filesystem>
#include <windows.h>
#include <ShlObj.h>
//...
std::filesystem::path GetAppDataPath() {
    namespace FS = std::filesystem;
    PWSTR ppszPath = nullptr;
    auto hr_path = ::SHGetKnownFolderPath(FOLDERID_RoamingAppData, KF_FLAG_DEFAULT, nullptr, &ppszPath);
    bool success = SUCCEEDED(hr_path);
    if (success) {
        auto p = FS::path(ppszPath);
        ::CoTaskMemFree(ppszPath);
        p = FS::canonical(p);
        return p;
    }
    return {};
}

Is there an equivalent for linux?

Linux is an operating system kernel. It does not have a concept of user directories.

There are several Linux distributions. The filesystem structure is determined by the distro. Most distros conform to POSIX standard, and follow (to varying degree) the Filesystem Hierarchy Standard by Linux Foundation, which is similar to the directory structures of other UNIX like systems. That said, distributions generally allow the user to use the file system in unconventional configurations. For example, they don't typically force users home directory to be under /home.

POSIX specifies a few environment variables that are relevant to this context:

HOME

The system shall initialize this variable at the time of login to be a pathname of the user's home directory.

TMPDIR

This variable shall represent a pathname of a directory made available for programs that need a place to create temporary files.

Environment variables can be accessed using std::getenv in C++.

On desktop systems, the directory structure is also determined to some degree by the desktop environment, of which there are several available. freedesktop.org produces unofficial specifications for interoperability of different desktop environments. On DE's conforming to XDG Base Directory Specification should following environment variables be available:

$XDG_DATA_HOME defines the base directory relative to which user specific data files should be stored. If $XDG_DATA_HOME is either not set or empty, a default equal to $HOME/.local/share should be used.

$XDG_CONFIG_HOME defines the base directory relative to which user specific configuration files should be stored. If $XDG_CONFIG_HOME is either not set or empty, a default equal to $HOME/.config should be used.

$XDG_DATA_DIRS defines the preference-ordered set of base directories to search for data files in addition to the $XDG_DATA_HOME base directory. The directories in $XDG_DATA_DIRS should be seperated with a colon ':'.

If $XDG_DATA_DIRS is either not set or empty, a value equal to /usr/local/share/:/usr/share/ should be used.

freedesktop.org also provides a utility xdg-user-dirs:

xdg-user-dirs is a tool to help manage "well known" user directories like the desktop folder and the music folder. It also handles localization (i.e. translation) of the filenames.

$(XDG_CONFIG_HOME)/user-dirs.dirs specifies the current set of directories for the user. This file is in a shell format, so its easy to access from a shell script. This file can also be modified by users (manually or via applications) to change the directories used.

So, in case of FOLDERID_RoamingAppData, you should probably use one of $XDG_x depending on the use case, falling back to the appropriate default relative to $HOME as specified.