使用 stl map 作为 struct 的成员

Question

I'm constructing a struct with one of the member being a map.
First question is this allowed? The compiler did not complain.

struct A {  
  map<int, float> B;  
}  

Later I declare an array of such data type.

A *C = (A *)INTERNAL_CALLOC(..., sizeof(A));  

Here the function INTERNAL_CALLOC is a functional wrapper of MALLOC.
Later on in the code when I try to first time insert an item into the array's first element's map, I got a core dump.

C[0].B[0] = 0.001;  

Any idea why is this the case?
Thanks!

Answer 1

Yes, a map in a struct is fine.

Allocating with malloc is definitely not fine; the constructor is not called. So your map will most likely do something terrible when you attempt to use it.

General rule of thumb: don't use malloc / calloc / realloc / free in C++. Avoid dynamic allocation wherever possible, and use new / delete when unavoidable. ^*

---

_{* And read up on smart pointers .}

Answer 2

If you really must allocate with INTERNAL_CALLOC, then use placement new.

First, you must define a constructor and a destructor for the type A, or define A as a class:

struct A {
  A(){}
  ~A(){}
  map<int, float> B;  
};

Then you can simply call:

//Allocate an A, uninitialized
A *C = (A *)INTERNAL_CALLOC(..., sizeof(A));  
//Initialize the newly allocated A
new (C) A();

later when you free the object you must explicitly call the destructor:

//A *C;
C->~A();
INTERNAL_free(C);

Answer 3

A *C = (A *)INTERNAL_CALLOC(..., sizeof(A));  

Here you are lying to the compiler. You are telling it that the return value from INTERNAL_CALLOC points to an A , but it doesn't, it just points to zeroes. Use new .

Answer 4

使用 malloc 分配内存不会初始化数组元素。

Answer 5

You don't get a valid std::map<whatever...> by filling memory with zero bytes.

To a certain extent, it's possible with POD types (approximately, "pure C" data structures).

Answer 6

Since you use std::map as a member, I recommend to use std::vector instead of plain array to be the container.

You can do as follow:

struct A
{
    std::map<int, int> B;
};

std::vector<A> vecofmap;    

A elem0;
A elem1;
A elem2;

vecofmap.push_back(elem0);
vecofmap.push_back(elem1);
vecofmap.push_back(elem2);

vecofmap[0].B[0] = 100;

std::cout << vecofmap[0].B[0] <<std::endl;

Then no needs to bother with memory allocation.