Question

I wrote the following example program but it crashes with segfault. The problem seems to be with using malloc and std::strings in the structure.

#include <iostream>
#include <string>
#include <cstdlib>

struct example {
 std::string data;
};

int main() {
 example *ex = (example *)malloc(sizeof(*ex));
 ex->data = "hello world";
 std::cout << ex->data << std::endl;
}

I can’t figure out how to make it work. Any ideas if it’s even possible to use malloc() and std::strings?

Thanks, Boda Cydo.

Answer 1

You can’t malloc a class with non-trivial constructor in C++. What you get from malloc is a block of raw memory, which does not contain a properly constructed object. Any attempts to use that memory as a “real” object will fail.

Instead of malloc-ing object, use new

example *ex = new example;

Your original code can be forced to work with malloc as well, by using the following sequence of steps: malloc raw memory first, construct the object in that raw memory second:

void *ex_raw = malloc(sizeof(example));
example *ex = new(ex_raw) example;

The form of new used above is called “placement new”. However, there’s no need for all this trickery in your case.