unsigned reference to unsigned long when compile c++ program

Question

I have a homework to deal with big number, but I cannot compile the most simple version of code. There are three files (.hh, .cpp and a test .cpp). Codes here: bignum.hh:

#pragma once

#include <cstdint>  // uint8_t

#include <iostream> // ostream

namespace bistro
{
    template <typename T = uint8_t>
    class BigNum
    {
    public:
        BigNum(std::size_t base);
    private:

    };
}

bignum.cpp:

#include "bignum.hh"

namespace bistro
{
    using std::size_t;

    // debug
    using std::cout;
    using std::endl;

    template<typename T>
    BigNum<T>::BigNum(size_t base) {
        cout << base << endl;
    }
}

test0.cpp:

#include "../src/bignum.hh"

using bistro::BigNum;

int main() {
    BigNum<uint8_t> *b = new BigNum<>(10);
    return 0;
}

and I use this command to compile them:

g++ -std=c++14 src/bignum.cpp tests/test0.cpp -o tests/test0

Here is the problem: enter image description here So, where is the problem?