Get space for array with malloc - c++

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:
mallocing non-POD types so their constructors don't run (catastrophic)
Failure to free the things you malloc (bad)
mallocing in C++ at all (not stylish)
Passing aaa* a[] when you meant aaa** a (valid but misleading)
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.