Get space for array with malloc

Question

Trying to pass pointer for array:

class aaa{
public:
    int a ;
    int b ;
    std::string c ;
};


void abc(aaa* a [])
{
    *a = (aaa*)malloc(sizeof(aaa)* 5);
    a[0]->c ="ddd" ;
    a[1]->c ="ccc" ;  //crash
    a[2]->c ="eee" ;
}

int main() {

    aaa * a;
    abc(&a);

    cout << "!!!Hello World!!!"<< a++->c << endl; 

    cout << "!!!Hello World!!!"<< a++->c << endl; 
    return 0;
}

On second array element assignment I have crash. Where is the problem? Does malloc not creates enough space?

UPD.

I can't change function void abc(aaa* a []) signature because of some reason. It is not mistakable signature even it looks not nice.

I have updated program according recomendations in answers, but I still have crash in getting second array element member:

cout << "!!!Hello World!!!"<< a[1].c << endl;

Why? What I do wrong in code below?

struct aaa{
public:
    int a ;
    int b ;
    std::string c ;
};


int abc(aaa* a [])
{
    int asize =5;
    *a = (aaa*)malloc(sizeof(aaa) * asize);
    for (int i;i<asize;i++)
    {
        a[i] = new aaa();
    }
    a[0]->c ="ddd" ;
    a[1]->c ="ccc" ;
    a[2]->c ="eee" ;
return asize;
}

int main() {
    aaa * a;
    int asize=abc(&a);

    cout << "!!!Hello World!!!"<< a[0].c << endl;
    cout << "!!!Hello World!!!"<< a[1].c << endl; //crash
    cout << "!!!Hello World!!!"<< a[2].c << endl;

    for (int i=0; i<asize;i++)
    {
        cout << "free "<<i<<endl;
        a[i].~aaa();
    }
    free(a);
    cout << "end"<<endl;
    return 0;
}

Answer 1

The problems are multifold:

1. mallocing non-POD types so their constructors don't run (catastrophic)
2. Failure to free the things you malloc (bad)
3. mallocing in C++ at all (not stylish)
4. Passing aaa* a[] when you meant aaa** a (valid but misleading)
5. No #includes, or namespace qualifier on cout and endl (invalid testcase)

Here's what your program should look like:

#include <vector>
#include <string>
#include <iostream>

class aaa
{
public:
    int a;
    int b;
    std::string c;
};


std::vector<aaa> abc()
{
    std::vector<aaa> result;
    result.reserve(3);

    result.push_back({0, 0, "ddd"});
    result.push_back({0, 0, "ccc"});
    result.push_back({0, 0, "eee"});

    return result;
}

int main()
{
    const auto a = abc();

    std::cout << "!!!Hello World!!!"<< a[0].c << std::endl;
    std::cout << "!!!Hello World!!!"<< a[1].c << std::endl;
    std::cout << "!!!Hello World!!!"<< a[2].c << std::endl;
}

(Live demo)

Or, to keep your five up-front element allocations:

std::vector<aaa> abc()
{
    std::vector<aaa> result(5);

    result[0].c = "ddd";
    result[1].c = "ccc";
    result[2].c = "eee";

    return result;
}

I strongly suggest forgetting everything you know about C, when you write C++.

Answer 2

You can use malloc, but you still need to call constructors, for this you need new, this defeats idea of using malloc.

const int asize = 5;

void abc(aaa*& a)
{
    a = (aaa*)malloc(sizeof(aaa) * asize); // you need to release memory later
    for (int i = 0; i < asize; ++i) {
        new (a+i) aaa(); // you need to call constructors to intialize string
    }
    // now you can use strings
    a[0].c = "ddd";
    a[1].c = "ccc";
    a[2].c = "eee";
}

int main() {
    aaa * a;
    abc(a);

    cout << "!!!Hello World!!!" << a[0].c << endl;
    cout << "!!!Hello World!!!" << a[1].c << endl;

    // finally you need to call destructors
    for (int i = 0; i < asize; ++i) {
        a[i].~aaa();
    }
    free(a);
    return 0;
}

After showing you how you can make it work, i would like to propose another solution. If you care about memory and don't want to use std::vector, you can use std::unique_ptr.

std::unique_ptr<aaa[]> data;
data = std::make_unique<aaa[]>(asize);
data[0].c = "text";
cout << data[0].c;
// no need to manually release memory

EDIT: As of updated question. If you really want to pass array of pointers then you can do the following:

const int asize = 5;

void abc(aaa* a[]) {
    // If array is really big, then you probably should preallocate memory and call placement new for every element.
    for (int i = 0; i < asize; ++i) {
        a[i] = new aaa; // again, you have to release memory 
    }
    // now you can use strings
    a[0]->c = "ddd";
    a[1]->c = "ccc";
    a[2]->c = "eee";
}

int main() {
    aaa * a[asize];
    abc(a);

    cout << "!!!Hello World!!!" << a[0]->c << endl;
    cout << "!!!Hello World!!!" << a[1]->c << endl;

    for (int i = 0; i < asize; ++i) {
        delete a[i];
    }
    return 0;
}

It would be very nice if you could use unique_ptr instead of raw pointer.