How to make a Currency class from std.BigInt?

[RuZzz via Digitalmars-d-learn]

module axfinance.api.currencies;

import std.array, std.stdio, std.system, std.bigint, std.conv;

class Currencies
{
        public:
                this(ulong pf)
                {
                        exponent(pf);
                }
                bool integer(BigInt pb)
                {
                        zInt = pb;
                        return true;
                }
                string value()
                {
                        string res = to!string(zInt);
                        insertInPlace(res, res.length - zExp,".");
                        return res;
                }
        private:
                bool exponent(ulong pe)
                {
                        zExp = pe;
                        return true;
                }
                BigInt zInt;
                ulong zExp;
}

unittest
{
        double d1 = 45.67;
        immutable LIM_FIAT = 2, LIM_EXCH = 6, LIM_CRYP = 8;
	auto c = [	"BTC":["N-01":new Currencies(LIM_CRYP), "N-02":new 
Currencies(LIM_CRYP), "SUM1":new Currencies(LIM_CRYP)],
			"CNY":["N-01":new Currencies(LIM_FIAT), "N-02":new 
Currencies(LIM_EXCH), "N-03":new Currencies(LIM_CRYP), "SUM1":new 
Currencies(LIM_CRYP), "SUM2":new Currencies(LIM_CRYP)]];
//       EUR, LTC, RUB;

	c["BTC"]["N-01"] = 1.00000002;//Error: cannot implicitly convert 
expression (1) of type double to 
axfinance.api.currencies.Currencies
        c["BTC"]["N-02"] = 15.00000002+"13455565.45665435";
        c["BTC"]["SUM1"] = c["BTC"]["N-01"] + c["BTC"]["N-02"];
        assert(c["BTC"]["SUM1"] == "13455581.45665437");
        c["CNY"]["N-01"] = 1.02;
        c["CNY"]["N-02"] = "0.01";
        c["CNY"]["N-03"] = "0.00000001";
        c["CNY"]["SUM1"] = c["CNY"]["N-01"] + c["CNY"]["N-02"];
        assert(c["CNY"]["SUM1"] == "1.03");
        assert(to!double(c["CNY"]["SUM1"]) == 1.03);
        c["CNY"]["SUM2"] = c["CNY"]["N-01"] + c["CNY"]["N-03"];
        assert(c["CNY"]["SUM2"] == "1.02000001");
}