传递std :: shared_ptr &lt; <std::vector<double> &gt;功能

Question

I am trying to understand the use of shared_ptrs. The simple code below

#include<iostream>
#include<vector>
#include<memory>

void funct(std::shared_ptr<std::vector<double>> H){
    H->push_back(1.00); 
}

int main()
{
std::shared_ptr<std::vector<double>> H;
funct(H);
return 0;
}

gives me a segmentation fault which I can't seem to understand. What am I doing wrong here?

Answer 1

H is initialized with a null pointer, so dereferencing it causes undefined behaviour.

You perhaps meant to do this:

auto H = std::make_shared<std::vector<double>>();

This would create a new empty std::vector<double> object owned by H . A pointer to an empty vector is quite different from a null pointer (which points to no vector at all).

Answer 2

Your smart pointer H is default created . When a smart pointer is default created, its pointer value is a nullptr .

So when you dereference it ( *H or H-> ), you will cause Undefined Behaviour .

In order to create a valid shared pointer, you can use the std::make_shared() function.

For example:

int main() {
  auto H = std::make_shared<std::vector<double>>();
  // Now H is not a nullptr pointer.
  funct(H);
  return 0;
}

(OT) Simple Advise Passing a shared_ptr by reference is probably not a good idea!

Answer 3

A shared pointer (or any smart pointer) behaves like a regular pointer in the sense that it is initially a nullptr, and after initialization will it start pointing at a specific memory address. In the case of a shared_ptr the initalization is best done with the make_shared function, which is described nicely in the reference if you need it.