will range based for loop in c++ preserve the index order

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别那么骄傲 2020-12-30 23:58

in c++11, if I use a range based for loop on vector, will it guarantee the iterating order? say, will the following code blocks are guaranteed for same output?

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长发绾君心 (楼主)

2020-12-31 00:47
Yes and no (It depends on the container in use):
- The range based for is a loop like for(iterator pos = range.begin(); pos != range.end(); ++pos) { /* with a range variable = *pos */ ... }
- An operator [] might do something different (eg. a std::map operator does a lookup on the key and create a new entry, if the key does not exist)
Example:
```
#include 
#include 

int main()
{
    typedef std::map map;
    map m = { { 0, 0 }, { 2, 2 }, { 4, 4 } };
    for(const auto& e : m) {
        std::cout << e.first << " ";
    }
    std::cout << std::endl;
    for(map::size_type i = 0; i < m.size(); ++i) {
        std::cout << m[i] << " ";
    }
    std::cout << std::endl;
    return 0;
}
```
The result is:
```
0 2 4 
0 0 2 0 4 
```
(The second result might be a good shot in the own foot or even intended)
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