Order of parameter pack expansion

Question

I have 2 functions to read binary file.

1st function reads sizeof(T) bytes from file:

template
T read() { ... some IO          


        

Answer 1

C++ does not specify the order in which a function's arguments are evaluated. If the expressions to a function all consume data from a stream, you can get behavior where objects are read in the wrong order.

Braced initializer lists are evaluated from left to right, however, so you should get better results if you try something like:

template<typename... Ts>
std::tuple<Ts...> read_all() {
    return std::tuple<Ts...>{read<Ts>()...};
}

Answer 2

I would keep it a bit simpler and do this:

uint32_t a;
uint8_t b;
std::tie(a, b) = read<std::tuple<uint32_t, uint8_t>>();

This way there's only a single read() and you can even skip the tie() if you use the tuple (or struct) fields directly.