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)

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