Can*t call chainActive.Tip()->nHeight in core.cpp

I want to call chainActive.Tip()->nHeight in core.cpp. It is not possible, I am interested in every idea.

https://github.com/LIMXTEC/BitSend/blob/DEV-joshafest/src/core.cpp

Here is an example :

bool GetH()
{   
    int nBlockHeight = chainActive.Tip()->nHeight;  
    if(nBlockHeight < FORKX17_Main_Net ) 
        return true;    
    else 
        return false; 
    }
}

uint256 CBlockHeader::GetHash() const 
{   
    if (GetH())     
    {
        return HashX11(BEGIN(nVersion), END(nNonce)); 
    }
    else    
    {
       return HashX17(BEGIN(nVersion), END(nNonce));    
    }
}

Also that is not possible:

CBlockIndex* pindexPrev = chainActive.Tip();
if (pindexPrev->nHeight <= FORKX17_Main_Net)