CODEWITHSUNDEEP
c++scopeshared-ptr
PinkElephantsOnParade
PinkElephantsOnParade

Reputation: 6592

Why does this shared_ptr code segfault? When do shared_ptr's free?

I've made a SSCE as best I can. My suspicion is that the shared pointers deconstruct (free) my objects before I ask for them in main. How can I prevent this without circumventing shared pointers altogether? This is in isolated problem in a program that otherwise is greatly helped by the use of shared_ptrs.

Test.h:

#ifndef TEST_H
#define TEST_H
#include <memory>
#include <iostream>
#include <vector>

class Test
{
    public:
        Test();
        virtual ~Test();
        static std::shared_ptr<Test> makeTestFrom(std::string l);
        std::vector<std::shared_ptr<int>> reg_vec;
    protected:
    private:

};

#endif // TEST_H

Test.cpp

#include <memory>
#include <iostream>
#include <vector>
Test::Test():
reg_vec()
{
    //ctor
}

Test::~Test()
{
    //dtor
}
std::shared_ptr<Test> Test::makeTestFrom(std::string l)
{
    std::shared_ptr<Test> sp(new Test());
    std::shared_ptr<int> i(new int(3));
    sp->reg_vec.push_back(i);
    return sp;
}

main.cpp:

#include <memory>
#include <iostream>
#include <vector>
#include "include/Test.h"
using namespace std;

 int main()
{
    std::unique_ptr<Test> x(new Test());
    x->makeTestFrom("loldoesntmatter");
    std::cout << x->reg_vec[0] << std::endl;
    return 0;

}

Upvotes: 1

Views: 1218

Answers (2)

Ben Voigt
Ben Voigt

Reputation: 283793

You're making too many new objects. In main, you're looking for an int inside x->reg_vec, but makeTestFrom doesn't add anything to x, it creates a brand-new object and puts the integer inside that.

Besides that, you're abusing shared_ptr. In C++, avoid dynamic allocation when you can. int are cheaper to pass by value than with shared_ptr, so just use vector<int>. And Test objects can be created with automatic lifetime also.

Just because some other languages (i.e. Java) make everything a handle doesn't mean that's a good pattern for C++.

Upvotes: 3

Anycorn
Anycorn

Reputation: 51545

 int main() {
    std::unique_ptr<Test> x(new Test());
    x->makeTestFrom("loldoesntmatter"); // you discarded the return
    std::cout << x->reg_vec[0] << std::endl; // x->reg_vec is empty
    return 0;

}

Also, too many shared pointers

Upvotes: 4

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