struct [[eosio::table]] _prices {
    symbol base;
    symbol quote;
    float value = 0.0;
    uint64_t primary_key() const {
        return base.code().raw();
    }
    uint128_t uniquekey() const {
        return ((uint128_t) base.code().raw()) << 64 | quote.code().raw();
    }
};

typedef eosio::multi_index<name("prices"), _prices,
    indexed_by<
        name("uniquekey"),
        const_mem_fun<
            _prices, uint128_t, &_prices::uniquekey
        >
    >
> prices;

symbol base("TTF", 2), quote("USD", 2);
float val = 1.0;
prices p(_self, _self.value);
auto m = p.get_index<name("uniquekey")>();
auto o = m.find((((uint128_t) base.code().raw()) << 64) | quote.code().raw());
if (o == m.end())
    p.emplace(_self, [&](auto &r) {
        r.base = base;
        r.quote = quote;
        r.value = val;
    });