std::vector::erase(item) needs assignment operator to be defined for item?

问题

I have a class C which does not define operator=. I have am trying to use a vector like so: std::vector<std::pair<C,D>> vec;. Now, my problem is that I am not able to erase the pair after I am done with it. The compiler complains of missing operator= for C. Can I not have a vector of a class which does not have this operator? How do I get around this? I cannot add assignment to C. This is the error I get:

error C2582: 'operator =' function is unavailable in 'C'    C:\...\include\utility  196 1   my-lib

This is my code:

void Remove(const C& c) 
{
    auto i = cs_.begin();
    while( i != cs_.end()) {
        if (i->first == c) {
            cs_.erase(i);  // this is the problem
            break;
        }
        i++;
    }
 }

where cs_ is:

std::vector<std::pair<C,D>> cs_;

回答1:

The reason is that erase will reallocate your objects if you erase a different position than std::vector::end(). A reallocation implicates copying.

Notice that a vector of an uncopyable type is only partially useable. Before we had emplace() (pre-C++11) it was even impossible. If your class is however copyable, why dont you define an assignment operator then?

A workaround could be a vector of smartpointers (or normal pointers) like std::vector<std::unique_ptr<C>>

回答2:

That's correct. The standard requires that items in a vector or deque be MoveAssignable if erase is to work.

In general, vectors and deques need to be able to erase from arbitrary positions. When they do, they need to be able to shift any remaining items into the space vacated by the erased item. The standard says (or implies, anyway, by its MoveAssignable requirement on erase) that they do that by assigning, so the contained item needs an assignment operator. Another option could have been for the items to be copied as by the copy constructor, but that's not how it's done.

来源：https://stackoverflow.com/questions/21310646/stdvectoreraseitem-needs-assignment-operator-to-be-defined-for-item