自定义容器与g ++（Rstudio）兼容，不适用于MSVS 2012

Question

I have a piece of C++ test code that works fine in Rstudio, which uses g++, but will result in run-time error in MSVS 2012:

template<typename T>
struct vec{
    T*head, *tail;
    vec()
    {
        head=NULL;
        tail=NULL;
    }
    vec(int n)
    {
        head=(T*)malloc(sizeof(T)*n);
        tail=head+n;
    }
    T operator [] (int i) const {return *(head+i);}
    T & operator [] (int i) {return *(head+i);}
    ~vec(){free(head);}
};

int main(){
    std::vector<int>y(3);
    y[0]=1; y[1]=2; y[2]=3;
    vec<int>x1(3);
    x1[0]=y[0];
    std::cout<<"vec of integers, [] overloading works fine \n";
    std::vector<std::vector<int>::iterator>z(3);
    z[0]=y.begin();
    z[1]=y.begin()+1;
    z[2]=y.begin()+2;
    vec<std::vector<int>::iterator>x2(3);
    x2[0]=z[0];
    std::cout<<*x2[0]<<"\n";
    std::cout<<"vec of std::vector::iterator, [] overloading g++ works fine, MSVS doesn't \n";
    return 1;
}

The code shows if vec contains integers, both MSVS and Rstudio work fine; if vec contains iterators, MSVS will run into the following error:

I sensed it has something to do with iterators. Could anyone let me know where my mistake is? Btw, I only included the vector header file in MSVS.

Thanks!

Answer 1

VS has additional error checking baked in to its iterators in debug mode. It found a legitimate bug in your code! The error is that you reinterpret uninitialized memory as initialized objects:

vec(int n){head=(T*)malloc(sizeof(T)*n);tail=head+n;}
                ^^^^^^^^^^^^^^^^^^^^^^^

T & operator [] (int i) {return *(head+i);}
                                ^^^^^^^^^

x2[0]=z[0];
^^^^^^

The assignment calls operator= on the vector iterator object, which is really uninitialized memory, which leads to the crash you're seeing when it tries to interpret some of that memory as valid data. (0xCDCDCDCD, by the way, is what the debugger commonly fills uninitialized memory with to help catch these kind of bugs quicker.)

I suggest using new / delete instead of malloc. Raw memory allocation is tricky to get right in C++ (where normal memory management is already plenty hard enough).

Note also that in the general case, malloc may not yield blocks with high enough alignment for all object types (although it will most of the time, since very few objects are aligned to more than 16 bytes).

Answer 2

You are allocating (uninitialized) memory using malloc and trying to interpret it as non-POD types like iterators can be. This is bound to fail - operator= for an object expects an initialized object as its left operand, but you are giving it a garbage value (hence the crash).

Also, you are not calling your elements' destructors, since you use plain free .

A simple solution is to use new T[n] in your constructor instead of malloc (and delete[] in the destructor). This requires your T to be default-constructible; there are ways to weaken this requirement (involving placement new and some tricks to guarantee alignment).

By the way, your vec class does not implement neither the "rule of three" nor the "rule of five", neither disables copy construction, so you are going to have double frees in case your vec object is ever copied.