An object can only be deallocated by delete if it was allocated by new and is not an array. If the argument to delete was not returned by new or is an array, the behavior is undefined.
An object can only be deallocated by delete[] if it was allocated by new and is an array. If the argument to delete[] was not returned by new or is not an array, the behavior is undefined.
If the argument to free was not returned by malloc, the behavior is undefined.
int* p1 = new int;
delete p1; // correct
// delete[] p1; // undefined
// free(p1); // undefined
int* p2 = new int[10];
delete[] p2; // correct
// delete p2; // undefined
// free(p2); // undefined
int* p3 = static_cast<int*>(malloc(sizeof(int)));
free(p3); // correct
// delete p3; // undefined
// delete[] p3; // undefined
Such issues can be avoided by completely avoiding malloc and free in C++ programs, preferring the standard library smart pointers over raw new and delete, and preferring std::vector and std::string over raw new and delete[].