Accessing vector items in GDB

Question

For example, I have such struct in a templated class:

struct Foo{
    int data;
    vector<Foo*> children;
}

And to print out the data value, I can simply do this: (let bar be a pointer to a Foo)

print bar->data

and this works fine. However I would like to also follow children to another Foo. I tried:

print bar->children[0]->data

but it doesn't work. How should I access the items in a vector and use it in print?

score 5 · Answer 1 · edited May 23 '17 at 11:51

5

With GDB 7.9 and g++ 4.9.2, it works quite well while printing bar->children[0]->data.

But, here is also an indirect method to access these elements: print (*(bar->children._M_impl._M_start)@bar->children.size())[0]->data where VECTOR._M_impl._M_start is the internal array of VECTOR and POINTER@VECTOR.size() is used for limiting the size of a pointer.

reference: How do I print the elements of a C++ vector in GDB?

Complement:

There is also another not so elegant but more general way:

print bar->children[0]

and you might get something like this:

(__gnu_cxx::__alloc_traits<std::allocator<Foo*> >::value_type &) @0x603118: 0x603090

so you can access to it with the pointer given above: print ((Foo)*0x603090).data

edited May 23 '17 at 11:51

Community

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answered Apr 12 '15 at 08:41

Tom

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I'm getting an error saying `There is no member or method named _M_impl.` Probably that's because I'm using clang++ instead of g++. – Derek 朕會功夫 Apr 12 '15 at 08:45
So can I know your environment? Because accessing to internal array of vector depends closely on implementationof compilers... – Tom Apr 12 '15 at 08:49
Clang++, libc++, C++14 with no optimization. – Derek 朕會功夫 Apr 12 '15 at 08:51
What's the output of print bar->children[0]? If you can get the pointer of this element, then you can print data field with "print ((Foo)*ADDRESS).data" – Tom Apr 12 '15 at 09:00
When I try to do `bar->children[0]` it just says `Could not find operator[]`. – Derek 朕會功夫 Apr 12 '15 at 09:09
Oh I just found another [solution](http://stackoverflow.com/a/24131961/283863) that made it worked by explicitly instantiating the vector. Thanks anyway. – Derek 朕會功夫 Apr 12 '15 at 09:27

Derek 朕會功夫 · Accepted Answer · 2020-12-02T13:43:59.773

2

With help from this answer, explicitly instantiating the vector fixes the problem.

For example,

template class std::vector<double>;

edited Dec 02 '20 at 13:43

answered Jun 28 '15 at 19:50

Derek 朕會功夫

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Can you give an example showing how it solved your problem? – Evandro Coan Dec 01 '20 at 21:06
@user Sure, I’ve included the snippet from the linked answer. – Derek 朕會功夫 Dec 02 '20 at 13:44
How would I use it in `gdb`? I would run: `gdb) template class std::vector;` and them `gdb) print bar->data` ? – Evandro Coan Dec 02 '20 at 13:52
IIRC I had to place it in the source code. – Derek 朕會功夫 Dec 03 '20 at 06:54

score 0 · Answer 3 · edited Jul 17 '23 at 12:54

0

template <> Foo* &std::vector<Foo*>::operator[](size_type n) noexcept
{
    return this->begin()[n];
}

to use vector operator[], need the Template materialization

edited Jul 17 '23 at 12:54

Delgan

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answered May 01 '22 at 02:52

Linhb

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Accessing vector items in GDB

3 Answers3