I'm a C++ beginner with a background in Python, Java, and JS, so I'm still learning the ropes when it comes to pointers.
I have a vector of shared pointers. Inside of a different function, I assign a shared pointer to a variable and add it to the vector. If I try to access the added element after that function exits, a segmentation fault happens:
class Bar
{
private:
std::vector<std::shared_ptr<Foo>> fooVector;
}
void Bar::addToFoo()
{
std::shared_ptr<Foo> foo (new Foo(…));
fooVector.push_back(foo);
}
void Bar::otherMethod()
{
// this method gets called sometime after addToFoo gets called
…
fooVector[anIndex]->baz(); // segfaults
…
}
But, if push_back a shared pointer and not a variable, it works.
// this works:
fooVector.push_back(std::shared_ptr<Foo>(new Foo(…)));
// this segfaults:
std::shared_ptr<Foo> foo (new Foo(…));
fooVector.push_back(foo);
I believe it happens because the foo
variable gets deleted when the addToFoo
function exits (correct me if I'm wrong). How do you push_back
a shared_ptr
variable to a vector
of shared_ptrs
in C++?
Why Use A Variable
Though pushing shared_ptr
s to vectors directly without variables works, I prefer to use variables in order to do this:
std::shared_ptr<Rider> rider;
switch (iProcessorModesParam)
{
case PEAKS_MODE:
rider = std::shared_ptr<Rider>(new PeaksRider(…));
break;
case RMS_MODE:
rider = std::shared_ptr<Rider>(new RMSrider(…));
break;
}
volumeRiders.push_back(rider);
PeaksRider and RMSrider are subclasses of Rider. I want to store all subtypes of Rider in the same vector of Riders. I learned that adding subtypes of Rider to a vector of Riders doesn't work and pointers are needed in order to achieve this kind of polymorphism:
std::vector<Rider> // doesn’t work with subtypes
std::vector<*Rider>
std::vector<std::shared_ptr<Rider>>
Having the std::shared_ptr<Rider> rider;
variable avoids repeating the .push_back(…)
code for each type of Rider.
Instead of assigning shared pointer, user reset
method.
rider.reset(new PeaksRider(…));
other that this, your code snippets seems to okay to me.
segfault may have caused because of the index variable ( which may be out of range). i suggest you to use .at(index)
for accessing pointer from vector and wrap that part of code in a try..catch
block and see what is the real error.
And regarding...
I believe it happens because the foo variable gets deleted when the addToFoo function exits (correct me if I'm wrong).
This is not true, share_ptrs use a local counter for #of references. as soon as you pushed the pointer to vector the counter gets incremented to 2 and event after control exits the function the counter is decremented to 1. so, your object is not destroyed yet.
There is no problem on creating a shared pointer instance, storing it in a variable, and doing a push_back to a vector after that. Your code should be fine as long as the index that you use when calling "otherMethod" is valid. However, I have a couple of suggestions for your code:
I just wrote a small snippet that you can test to illustrate what I just mentioned:
#include <iostream>
#include <vector>
#include <memory>
#include <cassert>
class Foo
{
public:
int data = 0;
};
class Bar
{
public:
void addNewFoo(int d)
{
std::shared_ptr<Foo> foo(new Foo());
foo->data = d;
fooVector.push_back(foo);
}
void addNewFooImproved(int d)
{
auto foo = std::make_shared<Foo>();
foo->data = d;
fooVector.push_back(foo);
}
void printFoo(int idx)
{
assert(idx < fooVector.size());
std::cout << fooVector[idx]->data << std::endl;
}
private:
std::vector<std::shared_ptr<Foo>> fooVector;
};
int main()
{
Bar b;
b.addNewFoo(10);
b.addNewFoo(12);
b.addNewFooImproved(22);
b.printFoo(1);
b.printFoo(2);
b.printFoo(0);
}
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