c++ - 如何修复此错误 `conversion from const_iterator to non-scalar type`？

Question

谁能解释一下这个错误是什么意思:

conversion from 'std::vector<int, std::allocator<int> >::const_iterator {aka __gnu_cxx::__normal_iterator<const int*, std::vector<int, std::allocator<int> > >}' to non-scalar type 'std::vector<int, std::allocator<int> >::iterator {aka __gnu_cxx::__normal_iterator<int*, std::vector<int, std::allocator<int> > >}' requested

给出以下类:

#include <vector>
#include <iostream>

using std::vector;
using std::ostream;

template<class T>
class Gen {
    vector<T> array;
public:
    explicit Gen(int size);
    template<class S>
    friend ostream& operator<<(ostream& os, const Gen<S>& g);
};

template<class T>
Gen<T>::Gen(int size) {
    for (int i = 0; i < size; i++) {
        this->array.push_back(T());
    }
}

template<class T>
ostream& operator<<(ostream& os, const Gen<T>& g) {
    for (typename vector<T>::iterator it = g.array.begin(); it != g.array.end();
            it++) { // ****** error ********
        os << *it << " ";
    }
    return os;
}

int main() {
    Gen<int> g(3);
    std::cout << g << std::endl;
}

我该如何解决？

Answer 1

您正在传递 const Gen<T>至 operator<< .这意味着当您调用 g.array.begin()您正在调用 begin 的 const 重载，它返回 const_iterator :

const_iterator begin() const noexcept;

然后您尝试将其分配给 vector<T>::iterator ，这会导致编译器错误。您可以这样解决:

auto it = g.array.begin()

它告诉编译器为 it 推断出正确的类型.