在 C++ Set 和 Vector 中取消引用迭代器时出错

Question

I'm writing this code and I'm getting this error:

[Error] passing 'const std::vector' as 'this' argument of 'void std::vector<_Tp, _Alloc>::push_back(const value_type&) [with _Tp = metastock7, _Alloc = std::allocator, std::vector<_Tp, _Alloc>::value_type = metastock7]' discards qualifiers [-fpermissive]

struct A{
     string name;
     vector<B> rows;
};
set<A, classcomp> set;
vector<B> data; //I filled the vector in my code
std::set<A, classcomp>::iterator it;
std::pair<std::set<A, classcomp>::iterator,bool> ret;
for(int i = 0; i < data.size(); i++){
    A a;
    B b = data[i];
    a.name= b.name;
    ret = set.insert(a);
    it = ret.first;
    (*it).rows.push_back(b); //IT COMPILES WITHOUT
    // it->rows.push_back(mstk7); //fails as well
}

I really don't understand the error. Can you please help?

Thank you.

Answer 1

std::set is an ordered container, so it doesn't allow you to directly modify its elements. If it did, you could invalidate its ordering guarantees.

To modify the element, you'd need to copy it, erase it from the set, modify it, then reinsert it. If you find yourself needing to do this often, you might want to consider using a different container type, especially as copying your std::vector member could get expensive.