How come std::distance() doesn't work for rvalues?-CodePudding

For example:

std::list <int> list1 = {1, 2, 3, 4, 5};
auto first = list1.begin();

std::cout <<  std::distance(--first,   first);

The output is 0. Strange enough, if we change --first to first--, the output becomes 5 (although here it should be 0, as it returns first). What am I missing?

CodePudding user response：

The order in which the arguments in a function call are evaluated is unspecified.

--first may execute first or first may execute first. If the former is the case, the decrement has undefined behavior, because decrementing a .begin() iterator is not allowed. Since one of the allowed execution orders has undefined behavior, your whole program has undefined behavior.

Using first-- instead doesn't change anything about this.

CodePudding user response：

Neither --first nor first is an rvalue. They are lvalues.

You are also not allowed to decrement the iterator returned by begin().

To get the distance between two rvalues, you could use std::prev and std::next:

auto second = std::next(list1.begin()); // to make std::prev(second) ok

std::cout <<  std::distance(std::prev(second), std::next(second));

CodePudding user response：

 std::distance(--first,   first)

This is undefined behavior. You are modifying the same variable twice. this is essentially the same as i i .