Would std::vector shrink to fit invalidate a reference to the vector itself?

Question

I want to know the precise meaning that
std::vector<T>::shrink_to_fit(new_capacity) would invalidate reference.

"If a reallocation happens, all iterators, pointers and references related to the container are invalidated. Otherwise, no changes." --

http://www.cplusplus.com/reference/vector/vector/shrink_to_fit/

Test code:

#include <vector>
#include <iostream>
class Test {
public:
    Test(const std::vector<int>& a) : _a(a) {}
    void print() const {
        std::cout << "[";
        for (auto x:_a) std::cout << " " << x;
        std::cout << " ]\n";
    }   
    const std::vector<int>& _a; 
};
int main() {
    std::vector<int> a;
    a.reserve(100);

    for (int i = 0; i < 10; ++i) a.push_back(i);
    Test t(a);
    t.print();
    a.shrink_to_fit();
    t.print(); // will this always work as expected?
}

If new_capacity is less than old_capacity (so the capacity of the vector shrinks), can the reference (the _a attribute of the test class) be invalidated?

Better reference: http://en.cppreference.com/w/cpp/container/vector/shrink_to_fit — Jesper Juhl, Jan 26 '17 at 21:15
cplusplus.com is full of mistakes so use one of the good websites like Cppreference.com — Raindrop7, Jan 26 '17 at 21:18
it seems that the war between the two sites would burn this account to ash. oh well. I just pick the top result of google. — hamster on wheels, Jan 26 '17 at 21:18
@rxu - You might want to start here [What is a reference variable in C++?](http://stackoverflow.com/questions/2765999/what-is-a-reference-variable-in-c). — Bo Persson, Jan 26 '17 at 21:20
@NicolBolas I wish there is a way to tell google not to show me this site when I search for C++ docs — Slava, Jan 26 '17 at 21:21
@Slava just start all your searches with `-site:cplusplus.com` so for example search for `-site:cplusplus.com shrink_to_fit` — Jerry Jeremiah, Jan 26 '17 at 21:57
@JerryJeremiah that's to much to type, easier to filter manually — Slava, Jan 26 '17 at 22:02

Mikel F · Accepted Answer · 2017-01-26T22:05:41.700

4

What the passage you are referring to means is that iterators or references to elements with the vector being resized can become invalidate.

In the case where _a is a reference, as your comment suggests, _a will continue to be a valid reference to the vector, but any iterators or references to elements within _a will be made invalid.

edited Jan 26 '17 at 22:05

answered Jan 26 '17 at 21:12

Mikel F

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my bad... i mean `std& _a;` – hamster on wheels Jan 26 '17 at 21:13
@rxu Edited to reflect that. – Mikel F Jan 26 '17 at 21:18
`_a is not a reference, but instead is a copy` but it is a reference in OP example – marcinj Jan 26 '17 at 21:54
@marcinj In the original version of his example it was not a reference, but you are correct in that I need to remove that part. – Mikel F Jan 26 '17 at 22:06

score 3 · Answer 2 · answered Jan 26 '17 at 21:13

Yes std::vector::shrink_to_fit() would invalidate references if reallocation occurs. But that unrelated to you sample code - references or pointers to data that vector holds would be invalidated, not reference to vector itself:

std::vector<int> v( 10 );
std::vector<int> &vref = v;
int &ref = v[0];
v.resize(2);
v.shrink_to_fit();
// now ref could be invalid, but vref is perfectly fine

score 3 · Answer 3 · answered Jan 26 '17 at 21:16

3

If new_capacity is less than old_capacity (so the capacity of the vector shrinks), can the reference (the _a attribute of the test class) be invalidated?

no, Test::_a will not be invalidated after:

a.shrink_to_fit();

if you would have a std::vector<int>::iterator _aitr;, which would point to some element of a, then such iterator would be invalidated.

answered Jan 26 '17 at 21:16

marcinj

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I would expect this is the case, because I think the vector is an object with a pointer to the actual memory that holds the data. shrink to fit might change this pointer, but a reference to the vector should still be fine. – hamster on wheels Jan 26 '17 at 21:17
@rxu exactly, that is true – marcinj Jan 26 '17 at 21:19

Would std::vector shrink to fit invalidate a reference to the vector itself?

3 Answers3