Get path to My Documents

It depends on how old of a system you need compatibility with. For old systems, there's SHGetSpecialFolderPath. For somewhat newer systems, there's SHGetFolderPath. Starting with Vista, there's SHGetKnownFolderPath.

Here's some demo code that works, at least on my machine:

#include <windows.h>
#include <iostream>
#include <shlobj.h>

#pragma comment(lib, "shell32.lib")

int main() { 
    CHAR my_documents[MAX_PATH];
    HRESULT result = SHGetFolderPath(NULL, CSIDL_PERSONAL, NULL, SHGFP_TYPE_CURRENT, my_documents);

    if (result != S_OK)
        std::cout << "Error: " << result << "\n";
    else
        std::cout << "Path: " << my_documents << "\n";
    return 0;
}

Use the SHGetFolderPath Windows API function and request CSIDL_MYDOCUMENTS.

Using Visual Studio 2017 with an MFC application under Windows 10 I am using the following code snippet with SHGetKnownFolderPath function to get the current user's Documents folder:

#include <string>     // include file for C++ native strings

//  . . .  other code.

PWSTR   ppszPath;    // variable to receive the path memory block pointer.

HRESULT hr = SHGetKnownFolderPath(FOLDERID_Documents, 0, NULL, &ppszPath);

std::wstring myPath;
if (SUCCEEDED(hr)) {
    myPath = ppszPath;      // make a local copy of the path
}

CoTaskMemFree(ppszPath);    // free up the path memory block

Note that the documentation has this to say about the path variable usage and the path returned:

ppszPath [out]

Type: PWSTR*

When this method returns, contains the address of a pointer to a null-terminated Unicode string that specifies the path of the known folder. The calling process is responsible for freeing this resource once it is no longer needed by calling CoTaskMemFree. The returned path does not include a trailing backslash. For example, "C:\Users" is returned rather than "C:\Users\".

For a list of the FOLDERID_ arguments possible see the MSDN article KNOWN_FOLDER_FLAG enumeration.