Question

Why does this crash? I did find out malloc() doesnt call constructors, so I called them myself manually, but it still crashes, I do not understand why.

PS. I know std::vector and new[] exists. Do not tell me to use vectors/new[] as an answer.

struct MyStruct {
    vector<int> list;
};
void make_crash(){
    MyStruct *array = (MyStruct *)malloc(100*sizeof(MyStruct));
    MyStruct element; // initialize element here since malloc() doesnt do it.
    array[0] = element; // copy, everything should be alright?
    array[0].list.push_back(1337); // nope, BANG!
    // The above line makes these:
    // First-chance exception at 0x7c970441 in test.exe: 0xC0000005: Access violation reading location 0xbaadf005.
    // First-chance exception at 0x00401cd0 in test.exe: 0xC0000005: Access violation reading location 0xbaadf00d.
    // Unhandled exception at 0x00401cd0 in test.exe: 0xC0000005: Access violation reading location 0xbaadf00d.
}

Answer 1

When you assign to a MyStruct

array[0] = element;

there is first an attempt to destroy the old members of the struct – but there isn’t any, because they were never constructed. Boom!

The easiest way to get a hundred MyStructs is to use another vector

vector<MyStruct>  v(100);

No need to use malloc.