Run time error while using realloc: “ _CrtIsValidHeapPointer(pUserData), dbgheap.c”

Question

The following code is written in C++ but using realloc from stdlib.h because I don't know much about std::vector.

Anyway, I get this weird run time error " " _CrtIsValidHeapPointer(pUserData), dbgheap.c".

If you would like to see the whole method or code please let me know.

I have 2 classes, Student and Grades. Student contains

char _name[21];         
char _id[6];             

int _numOfGrades;
int* _grades;
float _avg;

and Grade simply contains

Student* _students;
int _numOfStudents;

while the following works

_grades = (int *)realloc(_grades,(sizeof(int)*(_numOfGrades+1)));

this will create that weird run time error:

_students = (Student *)realloc(_students,(sizeof(Student)*(_numOfStudents+1)));

Both _grades and _students are created with new with no problem at all. The problem is only while trying to realloc _students.

Any input will be welcome.

Answer 1

You cannot mix allocators—if you allocate memory with operator new[] , you must deallocate it with operator delete[] . You cannot use free() , realloc() , or any other memory allocator (eg Windows' GlobalFree() / LocalFree() / HeapFree() functions).

realloc() can only reallocate memory regions which were allocated with the malloc() family of functions ( malloc() , calloc() , and realloc() ). Attempting to realloc any other memory block is undefined behavior—in this case, you got lucky and the C runtime was able to catch your error, but if you were unlucky, you might silently corrupt memory and then later crash at some random point in an "impossible" state.