Initializing a pointer to structure of typedefs

Question

I have read similar threads on SO on this topic (How to initialize a Pointer to a struct in C? and Initializing a pointer to a structure) but failed trying new or malloc. I've got a bunch of typedefs like

typedef void (*STP_CALLBACK_ON_TOPOLOGY_CHANGED) (BRIDGE* bridge); typedef void* (*STP_CALLBACK_ALLOC_AND_ZERO_MEMORY) (unsigned int size); typedef void (*STP_CALLBACK_FREE_MEMORY) (void* p); typedef void (*STP_CALLBACK_ENABLE_LEARNING) (BRIDGE* bridge, unsigned int portIndex, unsigned int treeIndex, bool enable);

which form a structure

 struct STP_CALLBACKS { STP_CALLBACK_ON_TOPOLOGY_CHANGED onTopologyChanged; STP_CALLBACK_ALLOC_AND_ZERO_MEMORY allocAndZeroMemory; STP_CALLBACK_FREE_MEMORY freeMemory; ... };

and when I want to init a pointer to this structure with

const STP_CALLBACKS *callbacks = new STP_CALLBACKS;

It actually inits nothing. I'm quite a newbie and obviously miss something important. What is a correct way to do it?

P.S. the code above is from mstp-lib. I'm on VS2015.

@doctorlove to use within a next function - STP_CreateBridge — Alexandr
– Alexandr, Commented Apr 10, 2017 at 10:44
You didn't tell it to initialise the contents. Since you declared it const, you'll have a hard time setting members unless you define a constructor. You don't need dynamic allocation. You just need to decide what functions to point at, or set them to nullptr. — paddy
– paddy, Commented Apr 10, 2017 at 10:51
I didn't ask why - I asked what. You need to tell it what the functions pointers should point to. If you show what functions you want it to point to, we can show you how. — doctorlove
– doctorlove, Commented Apr 10, 2017 at 11:12

Spock77 · Accepted Answer · 2017-04-10 11:23:08Z

Cause you are not actually initializing. Typedef just declares a type. All what new does above is allocating memory for 4 function pointers. To initialize this memory you need constructor:

struct STP_CALLBACKS { STP_CALLBACKS(STP_CALLBACK_ON_TOPOLOGY_CHANGED _fp1, ... ...

Then, having actual functions to be called like func1, func2, ... you can initialize your structure:

void func1(BRIDGE* bridge) { ... } STP_CALLBACKS callbacks(func1, ...

And then call back your functions:

BRIDGE bridge; callbacks.onTopologyChanged(&bridge);

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