Memory reallocation

[Ducky]

If I reallocate memory with 'new' it will erase what was written to the buffer but if I assign it to another pointer before reallocating, I can still print out what was there with the other pointer.
How is that possible when the second pointer points to the same address as the first?

int main()
{
    long iWholeSize = 17;
    char * buf = new (nothrow)char[iWholeSize];
    buf = "hello";
    char *pch = buf;

    buf = new (nothrow)char[iWholeSize+10];

    cout << "buf " << buf << "\n";
    cout << "pch " << pch << "\n";

    delete[] buf;
    return 0;
}

[vart]

If I reallocate memory with 'new' it will erase what was written to the buffer

It will not. you just replace you pointer value. but buffer still exists on the old address and the memory is "leaked" - since you have lost your access to it and have no way to free it.

[rcgldr]

This overwrites the pointer in buf:

    buf = "hello";

As far as I know, new does not reallocate memory. I get different addresses for buf and pch with this example:

int main()
{
    long iWholeSize = 17;
    char * buf = new (nothrow)char[iWholeSize];
    strcpy(buf, "hello");
    char *pch = buf;

    buf = new (nothrow)char[iWholeSize+10];
    strcpy(buf, "world");

    cout << "pch " << pch << "\n";
    cout << "buf " << buf << "\n";

    delete[] buf;
    delete[] pch;

    return 0;
}

[phantomotap]

O_o

I just wanted to elaborate on what vart has already said.

int main()
{
    long iWholeSize = 17;
    char * buf = new (nothrow)char[iWholeSize];
    buf = "hello"; // Leaks existing memory pointed to by `buf' without writing to the existing buf[n] because string literals don't work that way.
    char *pch = buf; // Simply creates a new pointer to the same memory without copying the memory pointed to by `buf' as pointers do not work that way.
 
    buf = new (nothrow)char[iWholeSize+10]; // Allocate memory without initializing the memory allocated to a known string.
 
    cout << "buf " << buf << "\n"; // You've told `cout' that `buf' is a null-terminated string, but you don't know what may be at the allocated memory.
    cout << "pch " << pch << "\n";
 
    delete[] buf;
    return 0;
}

[Ducky]

Thank you.

So should I delete[] buf the old pointer before reallocating it again?
If I delete it it will crash when I reallocate it.

[rcgldr]

So should I delete[] buf the old pointer before reallocating it again? If I delete it it will crash when I reallocate it.

new does not reallocate, it just allocates a new block of memory. You should either copy buf to another pointer (which you can delete[] later) or delete [] buf before doing another new with buf. If you want to resize, use C++ string or vector.

[Ducky]

I can't use string or vector cause my function takes a pointer, is it good this way?

int main()
{
    long iWholeSize = 17;
    char * buf = new (nothrow)char[iWholeSize];
    char pch[64];
    strcpy(pch, "hello");

    delete[] buf;
    buf = new (nothrow)char[iWholeSize+10];

    cout << "buf " << buf << "\n";
    cout << "pch " << pch << "\n";

    delete[] buf;
    return 0;
}

[phantomotap]

I can't use string or vector cause my function takes a pointer, is it good this way?

O_o

The example is incomplete as examples often are in that you aren't checking for null, and it also still has the problem of initializing `buf' to a string before handing the memory to `cout'.

You should check for `new' failing to allocate memory before you use it.

You should force the memory allocated by `buf' to be a valid null-terminated string within the memory you've allocated (`buf[0] = 0;').

Soma

[rcgldr]

I can't use string or vector cause my function takes a pointer, is it good this way? ... code example ...

In your example, the program allocates buf, deletes buf, allocates buf, deletes buf, but never puts anything into buf.

[laserlight]

I can't use string or vector cause my function takes a pointer

It may be possible that you can still use them, e.g.,

std::string buffer(iWholeSize);
foo(&buffer[0]);

[vart]

I can't use string or vector cause my function takes a pointer,

Still you can probably you std::string since it has a member c_str() which gives you a const pointer to c-style string contains in the object. and you can pass this pointer to your function. Of course your function should not try to modify contents of the pointer memory in this case

[vart]

It may be possible that you can still use them, e.g.,

std::string buffer(iWholeSize);
foo(&buffer[0]);

I thought only std::vector guarantees that pointer to first element will point to continues memory?

[rcgldr]

I can't use string or vector cause my function takes a pointer.

You could actually use a pointer to string:

int main()
{
    string * pstring = new string("init from cstring");
    cout << pstring->size() << ' ' << *pstring << endl;
    delete pstring;
    return 0;
}

[laserlight]

I thought only std::vector guarantees that pointer to first element will point to continues memory?

Changed for basic_string in C++11.

You could actually use a pointer to string:

That would probably be pointless though

[vart]

Changed for basic_string in C++11.

Good to know.
But looks like it is supported only starting with MSVC2012... I'm still on 2008/2010 mostly. Ducky is also not there yet.