c++ - 指向 vector 元素的持久指针代替 'this'

Question

我需要指向 std::vector 元素的持久指针。简单的 this 不会这样做，如下所示:

#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();
}

哪个产量

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

而我需要

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

当然，在这个例子中，可以很容易地变通。在现实生活中，持久指针应该传递给Qt::connect。

Answer 1

也许尝试像这样按值捕获索引:

#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();
}