c++ - 错误 : no matching member function for call to 'push_back'

Question

为什么我在最后两行收到错误？目标是在集合中找到对象，并修改其内容。

using namespace std;

struct mystruct {
  int id;
  vector<int> y;
  mystruct(const int id):id(id) {}
  bool operator<(const mystruct& x) const { return id < x.id; }
  bool operator==(const mystruct& x) const { return id == x.id; }
};

void test() {
  std::set<mystruct> sx;
  mystruct x(1);
  x.y.push_back(1); x.y.push_back(2);
  sx.insert(x);
  //
  set<mystruct>::iterator i = sx.find(1);
  const mystruct* x1 = &(*i);
  const mystruct x2 = *x1;
  cout << &(i->y) << endl;
  cout << &(x1->y) << endl;
  cout << x2.id << endl;
  x2.y.push_back(3);
  i->y.push_back(4);
}

好像迭代器返回的是常量对象，不让我用push_back()修改 vector y。我该如何克服这个问题？

错误:

test.cpp:27:8: error: no matching member function for call to 'push_back'
  x2.y.push_back(z);
  ~~~~~^~~~~~~~~
/opt/local/libexec/llvm-6.0/include/c++/v1/vector:688:36: note: candidate function not viable: 'this' argument has type 'const vector<int>', but method is not marked const
    _LIBCPP_INLINE_VISIBILITY void push_back(const_reference __x);
                                   ^
/opt/local/libexec/llvm-6.0/include/c++/v1/vector:691:36: note: candidate function not viable: 'this' argument has type 'const vector<int>', but method is not marked const
    _LIBCPP_INLINE_VISIBILITY void push_back(value_type&& __x);
                                   ^

Answer 1

自 x2用 const 声明限定符，即 const mystruct x2 , C++ 编译器只考虑 const -x2 上所有调用的合格成员函数及其任何成员。特别是，它正在寻找 void push_back (const int& val) const要调用的成员函数。显然，没有这个功能，因为push_back必须修改容器，所以编译器会产生一个错误来解释到底发生了什么:

candidate function not viable: 'this' argument has type 'const vector<int>', but method is not marked const

在您的代码中解决此问题的唯一方法是删除 const来自 x2 的限定符的声明。