Multi tool use

Clash Royale CLAN TAG

persistent pointer to vector element in place of 'this'

I need persistent pointers to elements of std::vector. Straightforward this won't do, as demonstrated here:

std::vector

this

#include <functional>
#include <vector>
#include <iostream>

class A {
public:
A() = delete;
A(int i)
: my_i(i)
, whoAmI([this]()->void{ std::cout<<"I am class A with i="<<my_i<<std::endl; })
{}
A(const A&) = delete;
A(A&&) = default;
int my_i;
std::function<void()> whoAmI;
};

int main()
{
std::vector<A> vec;
std::cout << "Initialization:" << std::endl;
for (int i=0; i<3; ++i) {
vec.push_back(A(i));
vec.back().whoAmI();
}
std::cout << "Retrieval:" << std::endl;
for (int i=0; i<3; ++i)
vec.at(i).whoAmI();
}


Which yields

Initialization:
I am class A with i=0
I am class A with i=1
I am class A with i=2
Retrieval:
I am class A with i=2
I am class A with i=2
I am class A with i=2


whereas I need

Retrieval:
I am class A with i=0
I am class A with i=1
I am class A with i=2


Of course, in this example one could easily work around. In real life, the persistent pointer shall be passed to Qt::connect.

Qt::connect

std::enable_shared_from_this

this

std::function

list

2 Answers
2

You are binding this at inizialization site of your lambda, so this is not going to work in in any circumstance the object is moved or copied indeed.

A possible solution could be to add a level of indirection:

#include <functional>
#include <vector>
#include <iostream>

class A;
class fuction_wrapper
{
public:
A* ref;
std::function<void(A*)> lambda;
void operator()() const { lambda(ref); }
};

class A {
public:
A() = delete;
A(int i)
: my_i(i)
, whoAmI({this, (A* a) { std::cout<<"I am class A with i="<< a->my_i << std::endl; }})
{}
A(const A&) = delete;
A(A&& a)
{
my_i = std::move(a.my_i);
whoAmI = { this, std::move(a.whoAmI.lambda) };
}

int my_i;
fuction_wrapper whoAmI;
};

int main()
{
std::vector<A> vec;
std::cout << "Initialization:" << std::endl;
for (int i=0; i<3; ++i) {
vec.push_back(A(i));
vec.back().whoAmI();
}
std::cout << "Retrieval:" << std::endl;
for (int i=0; i<3; ++i)
vec.at(i).whoAmI();
}


Probably not the most eye-candy solution available but it works.

Maybe try capturing indexes by value like this:

#include <functional>
#include <vector>
#include <iostream>


class A {
public:
A() = delete;
A(int i, std::vector<A> &vec)
: whoAmI([i, &vec]()->void{
std::cout<<"I am class A with i="<< i << ". Calling whoAmI(): " << std::endl;
vec[i];
})
{}
A(const A&) = delete;
A(A&&) = default;
std::function<void()> whoAmI;


};

int main()
{
std::vector<A> vec;
std::cout << "Initialization:" << std::endl;
for (int i=0; i<3; ++i) {
vec.push_back(A(i, vec));
vec.back().whoAmI();
}
std::cout << "Retrieval:" << std::endl;
for (int i=0; i<3; ++i)
vec.at(i).whoAmI();
}


Qt::connect

vec

vec[i]

A

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