CRT Doesn't print line number of memory leak

Question

I've got the code below, which I think, based on Finding Memory Leaks Using the CRT Library, should print out the line number of a memory leak.

#include "stdafx.h"
#define _CRTDBG_MAP_ALLOC
#include <stdlib.h>
#include <crtdbg.h>
#include <iostream>


void derp()
{
    int* q = new int;

}

int main()
{
    _CrtSetDbgFlag(_CRTDBG_ALLOC_MEM_DF | _CRTDBG_LEAK_CHECK_DF);
    derp();
    return 0;
}

When I run it, I get the following:

Detected memory leaks!
Dumping objects ->
{75} normal block at 0x0067E930, 4 bytes long.
 Data: <    > CD CD CD CD 
Object dump complete.

Based on Microsoft's documentation, I'd expect to see a print-out of the line where the leaky memory was allocated, but I don't.

What have I done wrong? I'm using VS2015.

See https://stackoverflow.com/q/3202520/4009370 – Chris Sprague Jul 25 '17 at 13:45 — Chris Sprague, Jul 25 '17 at 13:45

score 6 · Accepted Answer · edited Jul 26 '17 at 20:23

From the MSDN topic:

These techniques work for memory allocated using the standard CRT malloc function. If your program allocates memory using the C++ new operator, however, you may only see the file and line number where the implementation of global operator new calls _malloc_dbg in the memory-leak report. Because that behavior is not very useful, you can change it to report the line that made the allocation by using a macro that looks like this:

#ifdef _DEBUG
    #define DBG_NEW new ( _NORMAL_BLOCK , __FILE__ , __LINE__ )
    // Replace _NORMAL_BLOCK with _CLIENT_BLOCK if you want the
    // allocations to be of _CLIENT_BLOCK type
#else
    #define DBG_NEW new
#endif

And then replace the new in your code with DBG_NEW. I tested it and it works correctly with your code.

Actually, replacing the new with DBG_NEW everywhere in the code is too tedious task, so possibly your could use this macro:

#ifdef _DEBUG
     #define new new( _NORMAL_BLOCK , __FILE__ , __LINE__ )
#else
     #define new new
#endif

I tested this method and it works too.

good one.. but their offer is outdated, we can overload new https://blogs.msdn.microsoft.com/calvin_hsia/2009/01/19/overload-operator-new-to-detect-memory-leaks/ — Swift - Friday Pie, Jul 25 '17 at 13:53

score 2 · Answer 2 · answered Jul 25 '17 at 13:44

Check out the answer here. You want to use overloaded new operator with the additional parameters as specified in the solution there in order to get that information.

In this case, change your line

int* q = new int;

to

int* q = new (_NORMAL_BLOCK, __FILE__, __LINE__) int;

and you should see the leaks.

Swift - Friday Pie · Answer 3 · 2017-07-25T13:48:46.477

It should have been answered somewhere already.

"These techniques work for memory allocated using the standard CRT malloc function. If your program allocates memory using the C++ new operator, however, you need to redefine new if you want to see the file and line numbers in the memory-leak report."

In result you would have line of your new operator definition. you may use the trick that new can accept extra parameters and some of them can be defaulted to value of macrodefinitions, such like __LINE__ and __FILE__

CRT Doesn't print line number of memory leak

3 Answers3