Question

std::map<int, int> my_map;
my_map[0] = my_map.size();

Then, will my_map[0] be 0 or 1, or undefined?

Answer 1

There is no sequence point here, so the evaluation order is unspecified (or, in C++11 jargon, the two expressions are indeterminately sequenced).
The value of my_map[0] is 0 or 1 depending on the implementation.

• my_map[0] will increase the size if its evaluated first, causing my_map.size() to evaluate to 1. my_map[0] will then be 1.
• However, if my_map.size() is evaluated first, then the value for my_map[0] will be 0.

Now, how to make the above behavior well-defined? You must introduce a sequence point, that is, force one expression to be sequenced before the other; for behavior like the first,

int& val = my_map[0];
val = my_map.size();

… or, for behavior like the second,

int sz = my_map.size();
my_map[0] = sz;

Make sure to upvote Oo Tiib for being the first to demonstrate how to introduce sequencing for the expressions 🙂