a write function throwing the exception of read access violation in visual studio c++

I tried writing to a file and I am unable to write to it due to "Access violation reading location 0x0000000F" I am able to isolate the problem here is the sample code:

void test_1() {
    std::fstream fio{ "xyz.dat",std::ios::in | std::ios::out | std::ios::binary | std::ios::app };
    if (!fio) {
        std::cerr << "sorry no file";
        return;
    }
    std::string s_test{ "xyz hii \n workf" };
    fio.write( ( char* )(s_test.length()), sizeof( size_t ) );  //the write causing issue
}