无法正确推回结构C ++中的向量

Question

I currently have a struct p that holds a vector of another struct called pg. Here is the process struct:

struct p{
int i;
int s;
vector<pg*> pT;

void addP(struct pg* nP){
    pT.push_back(nP);
}

};

Somewhere else in the program, I create a new p struct by doing:

struct p *p1= (struct p*) malloc(sizeof(struct p));

and then assign values to id and size. I want to add a new page to the struct's pT vector. So I decide I can do p1-> addP(makeP()), where makePreturns a pointer to a pg struct, pg*.

Here is a link to what gdb shows up when it segfaults. It also mentions that it is happening on the push_back line. http://imgur.com/KZOGnI9

Is it something wrong with the pointers? Or am I not properly allocating memory for the vector?

Answer 1

Using malloc in C++ is almost always a bad idea. In your case, this leads to the constructor for struct process being not called, which in turn means the constructor of its member pageTable is not called, too.

Therefore, you're calling push_back on an uninitialized vector, resulting in undefined behavior (and in your case, a crash).

malloc presumably only works with "plain old datatypes", ie types that do not need any construction (have only a default constructor and only plain old datatypes as members).

The correct way to allocate objects on the heap in C++ is using new , which looks like this:

process *p = new process; // possibly add constructor arguments here

An even better way would be to use smart pointers, like std::unique_ptr or, if shared ownership is required, std::shared_ptr .

An even better approach is to not use pointers at all, especially for the objects in your vector, but probably also for your struct process . Use them only if you have to!

Answer 2

It is not correct to malloc your process object, since malloc won't call the constructor of the struct, which calls the constructor of std::vector<page*> .

If possible, make your process an ordinary automatic object:

process proc;

If you really need dynamic allocation, use new instead of malloc :

process* proc = new process;