[nsexception]

I'm trying to write my own container classes, and I need a resizeable C-style array. I am using malloc, realloc and free, but they never construct/destroy the arrays I am malloc/realloc/free-ing. I've gotten it to destroy (call the destructor) but never to construct (call the constructor). Here's my test code:

Code:
class temp {
public:
    temp() {
        printf("allocated\n");
    }
    ~temp() {
        printf("deallocated\n");
    }

};

int main (int argc, const char * argv[]) {
    //temp tem;

    temp * temps = malloc(sizeof(temp) * 4);

    allocator<temp> alloc ;
    for(int i = 0 ; i < 4 ; i++)
    {
        alloc.construct(&temps[i],temps[i]);
    }

    for(int i = 0 ; i < 4 ; i++)
    {
        alloc.destroy(temps+i);
    }
    
    free(temps);

    return 0;
}

It prints:

Code:
deallocated
deallocated
deallocated
deallocated

Thanks for any help.

Daniel

[Devin Lane]

I am not quite sure why it is not calling the constructor, but I believe it has to do with what the alloc.construct(pointer p, const Temp& val) function actually does, which is to call

Code:
 new(satic_cast<void*>(p)) T(val);

This is results in the compilier trying to convert from void* to temp*, which generates a warning. Also, when you are constructing the instances, you are initializing it with the value temps[i], which is probably empty or NULL. This might be causing the initialization to fail, but I am not sure.

I have a question for you though, why don't you use the new and delete operators directly, like this:

Code:
#include <iostream>

using namespace std;

class temp {
public:
    temp() {
        printf("allocated\n");
    }
    ~temp() {
        printf("deallocated\n");
    }

};

int main (int argc, const char * argv[]) {
    //temp tem;

    temp * temps = new temp[4];

    //do something with temps...

    delete[] temps;

    return 0;
}

You may have a reason for using alloc.construct and alloc.destroy, but I think new and delete will do what you want, unless you need really customized initialization.


Hope this helps

[nsexception]

Originally posted by Devin Lane:
You may have a reason for using alloc.construct and alloc.destroy, but I think new and delete will do what you want, unless you need really customized initialization.


Hope this helps

I would love to call new/delete, but you can't re-allocate! That's a pretty silly design point, so I have to use allocators.

Daniel

[BLAZE_MkIV]

why do you need to reallocate? just make a class


class MyContainer< class T> {
MyContainer( n ) {
call new
}
~MyContainer() {
call delete
}
resize( n ) {
new, then copy, then delete
}
};

BTW: slt gives you this for free and the C allocators while included are not garenteed to work with C++ objects. I'm surprised the destructor is even getting called.

[smeger]

If all you need is a dynamically-sized array, just use std::vector. The STL can be very useful.

Code:
#include <vector>

std::vector<int> myVector;
myVector.push_back(5);
myVector.push_back(10);
std::cout << myVector[0] << std::endl;

(Off the top of my head & I haven't used STL for awhile, treat code with care)

[BLAZE_MkIV]

That looks fine to me but he said he was writing his own container.

BTW my personal C++ bible is The C++ Programming Language by Stroustrup, Not much of a leaning book but if you finish one of those 30days book this has everything else that isn't platform dependent.

[Devin Lane]

C++ in a nutshell is also a great reference if you want to know everything (yes, this book is loaded) about c++. It contains complete library references, as well as discussion on topics like class, templates, etc.

[smeger]

I use C++, The Complete Reference, Vol. 3 by Schildt.

I'll have to try one of your recommendations, though. I'm not super-pleased with the one I've got. Stroupsoup'd probably be the way to go.