Question

I just read in the C++ standard that std::for_each is a non-modifying sequence operation, along with find, search and so on. Does that mean that the function applied to each element should not modify them? Why is that? What could possibly go wrong?

Here is a sample code, where the sequence is modified. Can you see anything wrong with it?

void foo(int & i) {     i = 12; }  int main() {     std::vector<int> v;     v.push_back(0);      std::for_each(v.begin(), v.end(), foo);     // v now contains 12 } 

I suspect this to be just an interpretation issue, but I wanted to have your opinion about that.

PS: I know I could use std::transform instead of for_each, but that’s not the point.

Answer 1

See this defect report they say

The LWG believes that nothing in the standard prohibits function objects that modify the sequence elements. The problem is that for_each is in a secion entitled ‘nonmutating algorithms’, and the title may be confusing. A nonnormative note should clarify that.

But also note this one.

They seem to call it ‘non-modifying’ because for_each itself does not exlicitly modify the elements of the sequence.