Detecting Windows 10 version

You can read real build number from the registry, and then infer Windows version from it. Your application does not need to have a manifest for this work: on my machine, it correctly detects OS build number as 10586. For example:

#include <Windows.h>
#include <sstream>

struct HKeyHolder
{
private:
    HKEY m_Key;

public:
    HKeyHolder() :
        m_Key(nullptr)
    {
    }

    HKeyHolder(const HKeyHolder&) = delete;
    HKeyHolder& operator=(const HKeyHolder&) = delete;

    ~HKeyHolder()
    {
        if (m_Key != nullptr)
            RegCloseKey(m_Key);
    }

    operator HKEY() const
    {
        return m_Key;
    }

    HKEY* operator&()
    {
        return &m_Key;
    }
};

bool IsRunningOnWindows10()
{
    HKeyHolder currentVersion;
    if (RegOpenKeyExW(HKEY_LOCAL_MACHINE, LR"(SOFTWARE\Microsoft\Windows NT\CurrentVersion)", 0, KEY_QUERY_VALUE, &currentVersion) != ERROR_SUCCESS)
        return false;

    DWORD valueType;
    BYTE buffer[256];
    DWORD bufferSize = 256;

    if (RegQueryValueExW(currentVersion, L"CurrentBuild", nullptr, &valueType, buffer, &bufferSize) != ERROR_SUCCESS)
        return false;

    if (valueType != REG_SZ)
        return false;

    int version;
    std::wistringstream versionStream(reinterpret_cast<wchar_t*>(buffer));
    versionStream >> version;

    return version > 9800;
}

The most straight-forward way to retrieve the true OS version is to call RtlGetVersion. It is what GetVersionEx and VerifyVersionInfo call, but doesn't employ the compatibility shims.

You can either use the DDK (by #including <ntddk.h> and linking against NtosKrnl.lib from kernel mode, or ntdll.lib from user mode), or use runtime dynamic linking as in the following snippet:

typedef LONG NTSTATUS, *PNTSTATUS;
#define STATUS_SUCCESS (0x00000000)

typedef NTSTATUS (WINAPI* RtlGetVersionPtr)(PRTL_OSVERSIONINFOW);

RTL_OSVERSIONINFOW GetRealOSVersion() {
    HMODULE hMod = ::GetModuleHandleW(L"ntdll.dll");
    if (hMod) {
        RtlGetVersionPtr fxPtr = (RtlGetVersionPtr)::GetProcAddress(hMod, "RtlGetVersion");
        if (fxPtr != nullptr) {
            RTL_OSVERSIONINFOW rovi = { 0 };
            rovi.dwOSVersionInfoSize = sizeof(rovi);
            if ( STATUS_SUCCESS == fxPtr(&rovi) ) {
                return rovi;
            }
        }
    }
    RTL_OSVERSIONINFOW rovi = { 0 };
    return rovi;
}

In case you need additional information you can pass an RTL_OSVERSIONINFOEXW structure in place of the RTL_OSVERSIONINFOW structure (properly assigning the dwOSVersionInfoSize member).

This returns the expected result on Windows 10, even when there is no manifest attached.

---

As an aside, it is commonly accepted as a better solution to provide different implementations based on available features rather than OS versions.

IsWindows10OrGreater() from VersionHelpers.h

Check the notes at Version Helper functions on MSDN

File VersionHelpers.h is shipped with Windows 10 SDK, but it will work in previous versions, too. Just copy it to your development environment.

It's just a header-only defined small lib, which uses VerSetConditionMask and VerifyVersionInfoW functions, both available in WinAPI since Windows 2000.

Upd If you can not include manifest file with your source code, you can use simple hack: just get a version of any system dll, for example, kernel32.dll using GetFileVersionInfo function.