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#include <iostream>
#include <vector>
 
class A
{
public:
    std::vector<int> data;
 
    A () = default;
    explicit A(const A&) = default;  // <---- accidental copies are avoided
 
    void DoTheThing()
    {
        while (data.size() < 10) {
            data.push_back(1);
        }
    }
};
 
class B
{
public:
    std::vector<A> objs;
 
    B()
    {
        A one, two, three;
        objs.push_back(one);
        objs.push_back(two);
        objs.push_back(three);
    }
 
    void DoTheThing()
    {
        for (auto obj: objs) {  // <---- error due to `explicit` copy consttructor
            obj.DoTheThing();
            std::cout << "DEBUG length during=" << obj.data.size() << std::endl;
        }
    }
};
 
int main()
{
    B b;
    b.DoTheThing();
    for (auto& obj : b.objs) {   // <--- OK
        std::cout << "DEBUG length  after=" << obj.data.size() << std::endl;
    }
 
    return 0;
}

prog.cpp: In member function 'void B::DoTheThing()':
prog.cpp:35:24: error: no matching function for call to 'A::A(A&)'
         for (auto obj: objs) {  // <---- error due to `explicit` copy consttructor
                        ^
prog.cpp:9:5: note: candidate: A::A()
     A () = default;
     ^
prog.cpp:9:5: note:   candidate expects 0 arguments, 1 provided

Standard output is empty