C++ 基于范围的 for 循环和元素副本

Question

I want to generate all subsets of a given vector:

vector<vector<int>> findSubsets(const vector<int> &nums) {
        vector<vector<int>> subsets;
        subsets.push_back(vector<int>{}); //empty set
        for (auto current_num : nums)
        {
            //add current_num to all existing subsets
            int n = subsets.size();
            for (int i = 0; i < n; i++) {
                vector<int> set(subsets[i]);
                set.push_back(num);
                subsets.push_back(set);
            }
        }
        return subsets;
    }

I wanted to use a range based for loop for iterating over all subsets it gives a different result:

vector<vector<int>> findSubsets(const vector<int> &nums) {
        vector<vector<int>> subsets;
        subsets.push_back(vector<int>{}); //empty set
        for (auto current_num : nums)
        {
            //add current_num to all existing subsets
            for (vector<int> subset : subsets)
            {
                subset.push_back(current_num);
                subsets.push_back(subset);
            }
        }
        return subsets;
    }

But this does not do what I intended. What am I missing?

Answer 1

As mentioned in the comment by @Fureeish one problem with the second version is inserting to the subsets vector while iterating over the subsets vector.

for (vector<int> subset : subsets)
 {
   subset.push_back(current_num);
   subsets.push_back(subset);  // Should not modify vector while iterating on it
 }

The range statement expands to something like:

{

    auto && __range = range_expression ;
    for (auto __begin = begin_expr, __end = end_expr; __begin != __end; ++__begin) {

        range_declaration = *__begin;
        loop_statement
    }
} 

The __end expression is expanded before the iterations. For a vector this could be calculating the address at vector.data[size]. If the loop pushes new elements into the vector, the __end expression is no longer correct.

If you iterated over a copy of the vector instead, the range statement would work. For example:

    for (vector<int> subset : vector<vector<int>>(subsets))
    {
        subset.push_back(current_num);
        subsets.push_back(subset);
    }