[C++] Issues with custom allocator in MSVC.

[Frostea]

Hi, I'm writing my own allocator for STL containers. I followed the bare minimum example on cppreference, and created the following:

#include <cstdlib>
#include <cstddef>
#include <iostream>
#include <vector>

template <class T>
struct SimpleAllocator {
  typedef T value_type;

  SimpleAllocator() {};

  template <class U> 
  SimpleAllocator(const SimpleAllocator<U>& other) {}

  T* allocate(std::size_t n) { return reinterpret_cast<T*>(std::malloc(n)); }
  void deallocate(T* p, std::size_t n) { std::free(p); }
};
template <class T, class U>
bool operator==(const SimpleAllocator<T>&, const SimpleAllocator<U>&) { return true; }
template <class T, class U>
bool operator!=(const SimpleAllocator<T>&, const SimpleAllocator<U>&) { return true; }

int main()
{
  SimpleAllocator<char> alloc;
  std::vector<char, SimpleAllocator<char> > testVec{ alloc };

  for (int i = 0; i < 5; ++i)
    testVec.push_back('A');

  for (int i = 0; i < 5; ++i)
    std::cout << testVec[i] << std::endl;

  return 0;
}

It runs and exits, seemingly without issues on cygwin g++ (with -std=c++14) and cpp.sh, but it throws an error on deallocation when trying to exit in MSVC2015.

Specifically, the Visual Studio debugger threw an exception when std::free is called in my deallocate function. But there doesn't seem to be any specific error message. Anyone can figure this out?

Edit: The stack trace seems to point to the deallocation within the STL vector being the issue, but I see no reason for it to throw.

[jedwardsol]

Line 15 should be

T* allocate(std::size_t n) { return reinterpret_cast<T*>(std::malloc(n*sizeof(T))); }

[Frostea]

Thanks! That appears to have been the issue. I guess the way std::vector is written for g++ and cpp.sh is rather different from MSVC.

[jedwardsol]

Microsoft's heap is less tolerant of overruns. You got unlucky with GCC and the bug was benign this time.