C++ getting error C2440

Question

Hello I'm using Visual Studio c++ 2010

I'm having a problem with this code ( it's taken from C language code ) :

MEMBLOCK* create_memblock (HANDLE hProc,  MEMORY_BASIC_INFORMATION *meminfo)
{

    MEMBLOCK *mb = malloc(sizeof(MEMBLOCK));

    if (mb)
    {
        mb->hProc = hProc;
        mb->addr = meminfo->BaseAddress;
        mb->size = meminfo->RegionSize;
        mb->buffer = malloc(meminfo->RegionSize);
        mb->next = NULL;

    }
    return mb;
}

I'm having these errors :

error C2440: 'initializing' : cannot convert from 'void *' to 'MEMBLOCK *'          
error C2440: '=' : cannot convert from 'PVOID' to 'unsigned char *'    
error C2440: '=' : cannot convert from 'void *' to 'unsigned char *'

I'm kinda newbie. Can you please provide a converted code for this that actually works with c++.

Thank you

malloc return `void*` you need to explicitly cast to `MEMBLOCK*`. Like this `MEMBLOCK *mb = (MEMBLOCK*)malloc(sizeof(MEMBLOCK));` or better `MEMBLOCK *mb = static_cast(malloc(sizeof(MEMBLOCK)));` — NetVipeC, Aug 07 '14 at 15:36
`MEMBLOCK *mb = (MEMBLOCK*)malloc(sizeof(MEMBLOCK));` or `/TC` option — BLUEPIXY, Aug 07 '14 at 15:39

score 4 · Answer 1 · answered Aug 07 '14 at 15:42

4

Since you're programming in C++, you should not use the old C function malloc. Instead I would recommend that you use the C++ new construct:

MEMBLOCK *mb = new MEMBLOCK;

answered Aug 07 '14 at 15:42

Some programmer dude

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score 3 · Accepted Answer · edited Jan 09 '18 at 11:02

3

In C++ You may not assign a pointer of type void * to a pointer of some other type. So for example instead of writing

MEMBLOCK *mb = malloc(sizeof(MEMBLOCK));

You have to write

MEMBLOCK *mb = ( MEMBLOCK * )malloc(sizeof(MEMBLOCK));

Also you have to change other statements where there is the same problem. It seems these statements are

mb->addr = ( unsigned char * )meminfo->BaseAddress;
mb->buffer = ( unsigned char * )malloc(meminfo->RegionSize);

It is a good example of that you always should use an explicit casting even in C. That makes the code more safe and clear.

edited Jan 09 '18 at 11:02

dnlkng

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answered Aug 07 '14 at 15:38

Vlad from Moscow

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Casting `void *` in C is neither safer nor clearer, it's completely redundant. The only advantage is polyglottism. – Quentin Jun 25 '18 at 08:05

score 2 · Answer 3 · edited Jun 25 '18 at 07:59

2

malloc() returns void*, and C++ does not automatically cast void* to a different pointer type. So you have to cast the return value:

MEMBLOCK *mb = (MEMBLOCK*) malloc(sizeof(MEMBLOCK));

Try:

MEMBLOCK* create_memblock (HANDLE hProc,  MEMORY_BASIC_INFORMATION *meminfo)
{
    MEMBLOCK *mb = (MEMBLOCK*)malloc(sizeof(MEMBLOCK));

    if (mb)
    {
        mb->hProc = hProc;
        mb->addr = meminfo->BaseAddress;
        mb->size = meminfo->RegionSize;
        mb->buffer = malloc(meminfo->RegionSize);
        mb->next = NULL;

    }
    return mb;
}

edited Jun 25 '18 at 07:59

Azeem

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answered Aug 07 '14 at 15:38

Sava B.

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You may want to say *why* the OP have to cast the return. Because, as we all know, in C you [should not cast the return of `malloc`](http://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc). – Some programmer dude Aug 07 '14 at 15:41

C++ getting error C2440

3 Answers3