C ++ for-each语句触发“向量迭代器不兼容”断言失败：this-> _Getcont（）== 0 [英] C++ for-each statement triggers "vector iterators incompatible" assertion failure: this->_Getcont() == 0

问题描述

这是使用Visual Studio 2012.

This is with Visual Studio 2012.

static void func(
  ...,
  const std::vector<std::string> &opt_extra_args_strs,
  ...)
{
   // THIS ASSERTS: "vector iterators incompatible"
   for (const std::string &arg_str : opt_extra_args_strs) {
      ... body does not modify opt_extra_args_strs

   // BUT THIS WORKS:
   for (size_t a_ix = 0; a_ix < opt_extra_args_strs.size(); a_ix++) {
       const std::string &arg_str = opt_extra_args_strs[a_ix];
}

我不是在循环体中修改向量，事实上，断言发生在第一次迭代之前。向量看起来在调试器，但我不知道足够STL来寻找腐败。在STL中，断言失败来自：

I am not modifying the vector at all in the loop body, and in fact, the assertion happens before the first iteration. The vector looks right in the debugger, but I don't know enough about STL to look for corruption. Inside the STL the assertion failure comes from:

void _Compat(const _Myiter& _Right) const {
    // test for compatible iterator pair
    if (this->_Getcont() == 0 // THIS FAILS (_Getcont() == 0)
         ...) {
        _DEBUG_ERROR("vector iterators incompatible");

与 this-> _Getcont（）因为（ _Myproxy 在 _Iterator_base12 中为NULL）。
调用堆栈是：

with this->_Getcont() being NULL because (_Myproxy is NULL in the _Iterator_base12). The call stack is:

msvcp110d.dll!std::_Debug_message(const wchar_t * message, const wchar_t * file, unsigned int line) Line 15 C++
Main.exe!std::_Vector_const_iterator<std::_Vector_val<std::_Simple_types<std::basic_string<char,std::char_traits<char>,std::allocator<char> > > > >::_Compat(const std::_Vector_const_iterator<std::_Vector_val<std::_Simple_types<std::basic_string<char,std::char_traits<char>,std::allocator<char> > > > > & _Right)
Main.exe!std::_Vector_const_iterator<std::_Vector_val<std::_Simple_types<std::basic_string<char,std::char_traits<char>,std::allocator<char> > > > >::operator==(const std::_Vector_const_iterator<std::_Vector_val<std::_Simple_types<std::basic_string<char,std::char_traits<char>,std::allocator<char> > > > > & _Right)
Main.exe!std::_Vector_const_iterator<std::_Vector_val<std::_Simple_types<std::basic_string<char,std::char_traits<char>,std::allocator<char> > > > >::operator!=(const std::_Vector_const_iterator<std::_Vector_val<std::_Simple_types<std::basic_string<char,std::char_traits<char>,std::allocator<char> > > > > & _Right)
Main.exe!run_test(..., const std::vector<std::basic_string<char,std::char_traits<char>,std::allocator<char> >,std::allocator<std::basic_string<char,std::char_traits<char>,std::allocator<char> > > > & opt_extra_args_strs)
    ...

我怀疑设置向量的代码正在以某种方式调整它，但我不确定。我也很难编写一个更简单的再现器，但程序应该是完全确定性的（单线程，而不是随机变量，总是断言）。

I suspect the code setting up the vector is screwing it up somehow, but I am not sure. I am having difficulty writing a simpler reproducer as well, but the program should be totally deterministic (single threaded, not random variables, always asserts).

此外，我也遇到了一个不同的类似的断言失败向量迭代器+偏移超出范围 （在同一向量上）

Additionally, I also run into a different similar assertion failure earlier "vector iterator + offset out of range" with the snippet (on the same vector)

template <typename T>
class Elsewhere {
    virtual void earlier(
        ....
        std::vector<T> &v) const
    {
       v.emplace_back(); // empty construction of a T
      // T &t = v.back(); // assertion failure
      T &val = to[to.size() - 1]; // but this works
      ... mutates val.

使用 T = std :: string 事实上，同一个向量）。

With T = std::string (in fact, the same vector).

我提到这一点，因为在STL中，这个失败的条件也是 this-> _Getcont ）== 0 ，我怀疑他们是相关的。 _Getcont（）在 Vector_const_iterator 中为0意味着什么？

I mention this because within STL the condition for this failure ends up also being this->_Getcont() == 0, and I suspect them to be related. What does it mean for _Getcont() to be 0 in a Vector_const_iterator?

向量来自容器

template <typename T>
struct type {
   T m_value;

   operator const T &() const {
     return value();
   }

   const T &value() const {
       return m_value;
   }
};

type<std::vector<std::string>> &t = ... method call that returns ref to it;
... t gets set
func(t); // implicit conversion to (const std::vector<std::string> &)

推荐答案

我终于找到了问题。向量的设置代码中的路径深深地破坏了向量的状态（将其设置为0，作为更大块内存的一部分）。这不会对向量中的前三个字段造成损害： _Myfirst ， _Mylast 和 _Myend ，因为这些字段在初始向量中为0。此外，大多数事情，如数组索引操作符和其他方法，如push_back仍然正确运行。但是，第四个字段 _Myproxy 最初不为零，并清除它禁用基于迭代器的功能。因此，对于每个循环， vector :: back（），其他所有失败与不同的错误，如伪边界检查，假不兼容的迭代器等...

I finally found the problem. A path deep in the vector's setup code was clobbering the vector's state (memset'ing it to 0's as part of a larger chunk of memory). This caused no harm for the first three fields in vector: _Myfirst, _Mylast, and _Myend since those fields are 0 in an initial vector. Moreover, most things such as the array index operator and other methods such as push_back still functioned correctly. However, the fourth field _Myproxy was initially non-zero, and clearing it disabled iterator-based functionality. Hence, for-each loop, vector::back(), and others all fail with different false errors such as false bounds checks, false incompatible iterators, etc...

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