[C++] Using malloc and new with struct

Let's say there is a struct defined as follow:

typedef struct StringBox_t {
    string str;
} StringBox;

A newbie may want to use malloc() to manage the memory like this:

StringBox *strBox = (StringBox *) malloc(sizeof(StringBox));
if (strBox == NULL) {
    exit(EXIT_FAILURE);
}
strBox->str = "That's fine."; // Actually it's not!
cout << strBox->str << endl;
free(strBox);

It's problematic because calling malloc() on a struct will not call the constructor of its member(s), which is a string in this case. So strBox->str = "That's fine."; winds up with a segmentation fault.

Now let's show the right way to handle this.

Using malloc() and free()

Besides calling malloc() on the struct, you should do that to every member of the struct if needed.

#include 
#include 
#include 

using namespace std;

typedef struct StringBox_t {
    string *p_str; // NOTE: we use a pointer here
} StringBox;

int main() {
    StringBox *strBox = (StringBox *) malloc(sizeof(StringBox));
    if (strBox == NULL) {
        exit(EXIT_FAILURE);
    }
    strBox->p_str = (string *) malloc(sizeof(string));
    if (strBox->p_str == NULL) {
        exit(EXIT_FAILURE);
    }

    *(strBox->p_str) = "That's fine finally.";
    cout << *(strBox->p_str) << endl;

    free(strBox->p_str);
    free(strBox);

    return 0;
}

Using new and delete

When you use new on a struct, the constructors of its members are automatically called. Therefore, you only need to use new once.

#include 
#include 
#include 
#include  // for std::nothrow

using namespace std;

typedef struct StringBox_t {
    string str; // NOTE: we don't use a pointer anymore here
} StringBox;

int main() {
    StringBox *strBox = new(nothrow) StringBox;
    if (strBox == NULL) {
        exit(EXIT_FAILURE);
    }

    strBox->str = "That's fine too.";
    cout << strBox->str << endl;

    delete strBox;

    return 0;
}

Note that if new fails, instead of returning a NULL pointer, an exception is thrown. To handle this, new(std::nothrow) should be used.