c++ string in C struct, is it illegal?

Question

struct run_male_walker_struct {
        string male_user_name;
        string show_name;
};
typedef struct run_male_walker_struct run_male_walker_struct_t;

in another function:

run_male_walker_struct_t *p = malloc(sizeof(struct run_male_walker_struct));

question, is it illegal? As the string is a class, it's size can't be determined by sizeof().

Answer 1

This is illegal, but not for the reasons you're thinking.

The difference between std::malloc()/std::free() and new/delete is that the latter will call constructors/destructors, while the former won't. The expression

void* p = std::malloc(sizeof(run_male_walker_struct))

will return a blob of uninitialized memory on which no constructor is called. You shouldn't touch it with a ten foot pole - except for invoking a constructor on it:

run_male_walker_struct* pw = new(p) run_male_walker_struct;

If you do this, you will have to do the reverse, too:

pw->~run_male_walker_struct();

before you free the memory:

std::free(p);

However, that leaves the question why you want to do that.
The only reason to do this should be when you want to separate memory allocation from construction (like, for example, in a pool allocator). But if you need that, it's best hidden behind some interface. A natural one would be overloading new and delete per class. Also, std::vector does this internally.