如何在 C++ Lambda 的向量中捕获元素？

Question

Scenario:

I initialized an instance with a Lambda in it which the Lambda can modify the instance state through capturing the instance by reference. When I put the instance in a vector and invoke the Lambda, the Lambda modifies the original initialized instance not the instance in the vector.

Minimal C++ Program to illustrate:

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

struct BoolState {
    bool _stateChangeByInstanceReference = false;
    bool _stateChangeByContainerReference = false;
    std::function<void()> _changeStateByInstanceReference;
    std::function<void()> _changeStateByContainerReference;
};

int main()
{
    using namespace std;
    vector<BoolState> bss;
    BoolState bs;
    bs._changeStateByInstanceReference = [&bs]() -> void { bs._stateChangeByInstanceReference = true; };
    bs._changeStateByContainerReference = [&bss]() -> void { bss.at(0)._stateChangeByContainerReference = true; };
    bss.emplace_back(bs);
    bss.back()._changeStateByInstanceReference();
    bss.back()._changeStateByContainerReference();
    cout << "The original instance by instance reference: " << bs._stateChangeByInstanceReference << endl;
    cout << "The original instance by container reference? " << bs._stateChangeByContainerReference << endl;
    cout << "The container instance by instance reference? " << bss.back()._stateChangeByInstanceReference << endl;
    cout << "The container instance by container reference: " << bss.back()._stateChangeByContainerReference << endl;
    /*
        The original instance by instance reference: 1
        The original instance by container reference? 0
        The container instance by instance reference? 0
        The container instance by container reference: 1
    */
}

Question:

• Since I captured the instance by reference in Lambda, how can the original initializied instance and the instance in vector are 2 different instance? (I can print that the 2 instances have 2 different address in memory, but I can't understand: why is this case? and how would this influence the result?)
• Can I modify the container instance state by the Lambda capturing the original reference and vice versa?

Solution according to @Asteroids With Wings, @idclev 463035818 and @Caleth:

Thank you all very much. This helps me a lot to understand what's going on here and how to fix it.

push_back() version according to @Asteroids With Wings:

int main()
{
    using namespace std;
    vector<BoolState> bss;
    BoolState bs;
    bs._changeStateByContainerReference = [&bss]() -> void { bss.at(0)._stateChangeByContainerReference = true; };
    // Only copy container version Lambda
    bss.push_back(bs);
    // Modify in place with instance version Lambda
    BoolState& bsInbss = bss.at(0);
    bss.at(0)._changeStateByInstanceReference = [&bsInbss]() -> void { bsInbss._stateChangeByInstanceReference = true; };
    bss.at(0)._changeStateByInstanceReference();
    bss.at(0)._changeStateByContainerReference();
    cout << "The container instance by instance reference:(solved) " << bss.back()._stateChangeByInstanceReference << endl;
    cout << "The container instance by container reference: " << bss.back()._stateChangeByContainerReference << endl;
}

"real" emplace_back() version accroding to @idclev 463035818

int main() {
    using namespace std;
    vector<BoolState> bss;
    bss.emplace_back(BoolState());
    BoolState& bsInbss = bss.at(0);
    bss.at(0)._changeStateByInstanceReference = [&bsInbss]() -> void { bsInbss._stateChangeByInstanceReference = true; };
    bss.at(0)._changeStateByContainerReference = [&bss]() -> void { bss.at(0)._stateChangeByContainerReference = true; };
    bss.at(0)._changeStateByInstanceReference();
    bss.at(0)._changeStateByContainerReference();
    cout << "The container instance by instance reference:(solved) " << bss.back()._stateChangeByInstanceReference << endl;
    cout << "The container instance by container reference: " << bss.back()._stateChangeByContainerReference << endl;
}

Accroding to @Caleth, I did not try to modify the original instance again, feel free to try youself if you are still interested in referring each other method.

Answer 1

When I put the instance in a vector

You can't do that. You can copy the instance into a vector element. In C++ an object is the memory it occupies.

Can I modify the container instance state by the Lambda capturing the original reference and vice versa?

Because they are separate objects, they would need some way of referring to each other, such as a BoolState * other member.

Answer 2

Emplacing an element into a container does construct the element in place. The constructor is called with the parameters provided by you. It helps to avoid creating an unnecessary instance when all you need is to copy/move it into the container.

However, if you do have an instance already, then emplacing an element is not much different from pushing it: The parameter is used to call the copy constructor and you end up with a copy in the container.

Terminology is perhaps a little misleading, because you can never first create an object and then place that object in a container. Consider the simple case of a c-array:

int x;
int a[3];

There is no way to have &x == &a[0] , ie an element inside the container can be equal, but not identical to an element outside. Of course you can emulate it with a level of indirection (eg pointers).

Answer 3

Here's the real minimal example:

#include <iostream>
#include <vector>

int main()
{
    std::vector<int> v;
    
    int x = 42;
    v.emplace_back(x);
    
    std::cout << &x << '\n';
    std::cout << &v[0] << '\n';
}

// 0x7ffd008d9d14
// 0x1f7fc20

( live demo )

Vectors own their contents. They do that by copying what you give them.

Your object in main is not the same object that's in the vector. Modifying one will not modify the other.

That doesn't mean you can't do what you want, but you'll have to give the object that's in the vector a name in order to capture it:

BoolState& bsInVector = bss.at(0);
auto func = [&bsInVector]() { /* ... */ };