Macro to replace C++ operator new

Question

Is it possible to create macros to replace all forms of operator new with overloads that include additional args...say __FILE__ and __LINE__<

Answer 1

Here is what I use:

In new.cpp

const char* __file__ = "unknown";
size_t __line__ = 0;

void* operator new(size_t size) {
    void *ptr = malloc(size);
    record_alloc(ptr,__file__,__line__);
    __file__ = "unknown";
    __line__ = 0;
    return ptr;
}

void delete(void *ptr)
{
   unrecord_alloc(ptr);
   free(ptr);
}

For compactness, I'm leaving out the other definitions of new and delete. "record_alloc" and "unrecord_alloc" are functions that maintain a linked list of structure containing ptr, line, and file).

in new.hpp

extern const char* __file__;
extern size_t __line__;
#define new (__file__=__FILE__,__line__=__LINE__) && 0 ? NULL : new

For g++, "new" is expanded only once. The key is the "&& 0" which makes it false and causes the real new to be used. For example,

char *str = new char[100];

is expanded by the preprocessor to

char *str = (__file__="somefile.c",__line__=some_number) && 0 ? NULL : new char [100];

Thus file and line number are recorded and your custom new function is called.

This works for any form of new -- as long as there is a corresponding form in new.cpp

Answer 2

No, there's no way.

You could do this in the bad old days of malloc()/free() but not for new.

You can replace the memory allocator by globally overriding the new operator, but you cannot inject the special variables you're talking about.

Answer 3

You should check out this excellent blog entry by my coworker Calvin. We had a situation recently where we wanted to enable this type of fix in order to associate memory leaks with the line that allocated them in diagnostic/debug builds. It's an interesting trick

https://docs.microsoft.com/en-us/archive/blogs/calvin_hsia/overload-operator-new-to-detect-memory-leaks

Answer 4

You don't say what compiler you are using, but at least with GCC, you can override new and log the caller address, then later translate that to file/line information with addr2line (or use the BFD library to do that immediately).

Answer 5

What you could do is to overload the operator new and get the stack trace there (platform specific) and use the stack information to deduce from where new was called.

Answer 6

I found the following library "nvwa" very useful for tracking down new/delete memory leaks - have a look at the file "debug_new" for examples, or just use it 'as is'.