1

Trying to make vector from part(5 elements) of c-array.

const static int size = 10;
cout << "vector from C-array: " << endl;
int ia[size] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
vector<int> vi2(ia, ia + size-5);

But when I try enumerate vector items using whole length of c-array I got all items like vector would be initialized like vector<int> vi2(ia, ia + size-5); . No exception or error that it goes out of range. 

    for(int i = 0; i < size; i++) {
    cout << i << ":" << vi2[i] << " ";
}

In output:

0:1 1:2 2:3 3:4 4:5 5:6 6:7 7:8 8:9 9:10

Why vector initialization is using second param that describes pointer to array end if it doesn't uses it?

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5
  • Have you checked vi2.size()? It should give 5, and if it does, accessing beyond the 5th element will give you undefined behaviour. Commented Dec 21, 2013 at 16:53
  • vi2 is using that end of array pointer. You are the one who is ignoring it and indexing it all the way to size. Commented Dec 21, 2013 at 16:54
  • If you want an exception to be thrown, use vector<T>::at() instead of vector<T>::operator[].\ Commented Dec 21, 2013 at 16:57
  • The output shown is possible, like it's possible to get an asteroid in the head. It is however just as unlikely. Can you post a complete but minimal example that exhibits the claimed behavior. Commented Dec 21, 2013 at 17:05
  • BTW: If you want five elements, write vector<int> vi2(ia, ia + 5); instead of vector<int> vi2(ia, ia + size-5);. In this case, it doesn't matter, because size-5 == 5, but the first way indicates that you want the first five, and the second indicates you want all but the last five. Commented Dec 21, 2013 at 17:51

4 Answers 4

Reset to default
3

You are accessing your vector out of bounds. This is undefined behaviour. There is no requirement that an exception be thrown.

You must not go beyond vi2.size()-1.

for(int i = 0; i < vi2.size(); ++i) 
{
  std::cout << i << ":" << vi2[i] << " ";
}
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2

The [] operator doesn't do any bounds checking (similarly to the [] operator in C and C++ which works with raw arrays).

If you want an exception to be thrown, then use the at member function:

std::vector<int> v { 1, 2, 3, 4, 5 };
int invalidInt = v.at(5); // hell breaks lose, says the runtime
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1

C++ doesn't throw an exception the way Java does if you access an out of bounds index. It's undefined behavior, and you simply must not do it.

for(int i = 0; i < vi2.size(); i++) {
    cout << i << ":" << vi2[i] << " ";
}

See A Guide to Undefined Behavior in C and C++:

In a safe programming language, errors are trapped as they happen. Java, for example, is largely safe via its exception system. In an unsafe programming language, errors are not trapped. Rather, after executing an erroneous operation the program keeps going, but in a silently faulty way that may have observable consequences later on.

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3 Comments

Agree regarding exception, but vector initialization looks still stupid for me. Why C++ needs to copy whole array in to vector instead copying just 5 elements
It doesn't. I doubt most compilers do that, since it's so inefficient.
@user1501700 it only copies 5 elements.
1

If you really want an exception to be thrown when you're out of bounds, use .at():

for(int i = 0; i < size; i++) {
    cout << i << ":" << vi2.at(i) << " ";
}

That being said, better use the actual size of vi2 (vi2.size()) instead.

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