Difference between std::pair and std::tuple with only two members?

Question

Is there a difference between an std::pair and an std::tuple with only two members? (Besides the obvious that std::pair requires two and only two members and tuple may have more or less...)

Answer 1

There are some differences:

1. std::tuple can never be by standard-layout (at least, it's not required to be by the standard). Every std::pair<T, Y> is standard-layout if both T and Y are standard-layout.

2. It's a bit easier to get the contents of a pair than a tuple. You have to use a function call in the tuple case, while the pair case is just a member field.

But that's about it.

Answer 2

This is a very late answer but note that, because std::pair is defined with member variables, its size cannot be optimized using empty base class optimization (first and second must occupy distinct addresses, even if one or both is an empty class). This exacerbated by whatever alignment requirements second_type has, so in the worst case the resulting std::pair will be basically twice the size it needs to be.

std::tuple only allows access through helper functions, so it's possible for it to derive from either type if one or the other is empty, saving on the overhead. GCC's implementation, at very least, definitely does this...you can poke through the headers to verify this but there's also this as evidence.

Answer 3

An std::tuple's name is longer (one extra character). More of those characters are typed with the right hand, so easier for most people to type.

That said, std::pair can only have two values - not zero, one, three or more. TWO values. A tuple, however, has almost no semantic limitation on the number of values. An std::pair, therefore, is a more accurate, type safe type to use if you actually want to specify a pair of values.

Answer 4

For what it's worth, I find the GDB output of std::tuple to be far more difficult to read. Obviously if you need more than 2 values then std::pair won't work, but I do consider this a point in favor of structs.

Answer 5

Note that with C++ 17, one can use the same interface to read data from both pair and tuple with two elements.

auto [a, b] = FunctionToReturnPairOrTuple();

No need to use get<> :)