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?

Ammoniacal

Answer 1

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

For example,

template class std::vector<double>;

Ammoniacal

Can you give an example showing how it solved your problem? – Retrospect

@user Sure, I’ve included the snippet from the linked answer. – Vanhoose

How would I use it in gdb? I would run: gdb) template class std::vector<double>; and them gdb) print bar->data ? – Retrospect

IIRC I had to place it in the source code. – Vanhoose

Answer 2

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 [email protected]() 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

Sirkin

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++. – Vanhoose

So can I know your environment? Because accessing to internal array of vector depends closely on implementationof compilers... – Sirkin

Clang++, libc++, C++14 with no optimization. – Vanhoose

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" – Sirkin

When I try to do bar->children[0] it just says Could not find operator[]. – Vanhoose

Oh I just found another solution that made it worked by explicitly instantiating the vector. Thanks anyway. – Vanhoose

Answer 3

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

to use vector operator[], need the Template materialization

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