Compiler errors with incorrect use of nullptr - c++

Question

I am trying the solution provided in this SO Q/ACompiler error while using shared_ptr with a pointer to a pointer and I am not able to use the solution provided in a proper way. I still get compilation errors on Ubuntu 18.04 with g++ version 7.3
Here is my minimum complete verifiable example to reproduce the problem
test.h
# include <memory>
using std::shared_ptr;
using std::unique_ptr;
struct DataNode
{
shared_ptr<DataNode> next;
} ;
struct ProxyNode
{
shared_ptr<DataNode> pointers[5];
} ;
struct _test_
{
shared_ptr<shared_ptr<ProxyNode>> flane_pointers;
};
test.cpp
#include <stdint.h>
#include "test.h"
shared_ptr<DataNode> newNode(uint64_t key);
shared_ptr<ProxyNode> newProxyNode(shared_ptr<DataNode> node);
struct _test_ test1;
int main(void)
{
test1.flane_pointers(nullptr);
shared_ptr<DataNode> node = newNode(1000);
}
shared_ptr<ProxyNode> newProxyNode(shared_ptr<DataNode> node) {
shared_ptr<ProxyNode> proxy(new ProxyNode());
return proxy;
}
shared_ptr<DataNode> newNode(uint64_t key) {
shared_ptr<DataNode> node(new DataNode());
return node;
}
This is the error I get
test.cpp: In function ‘int main()’:
test.cpp:11:31: error: no match for call to ‘(std::shared_ptr<std::shared_ptr<ProxyNode> >) (std::nullptr_t)’
test1.flane_pointers(nullptr);
^
What else have you tried ?
I tried initializing the nullptr in the header file as well
struct _test_
{
shared_ptr<shared_ptr<ProxyNode>> flane_pointers(nullptr);
};
But that did not work either. Where am I going wrong ?
My Goal
All I am trying to do is the following - I am trying to initialize flane_pointers which is a vector of pointers to a nullptr. The declaration has been made in a header file as to what type it is and I am trying to initialize it in a .cpp file. While doing that I get the above compilation errors.
flane_pointers(nullptr)
UPDATE
Could any of the answers explain whether the initialization provided in this Compiler error while using shared_ptr with a pointer to a pointer correct or not ?
std::shared_ptr<std::shared_ptr<ProxyNode> > ptr2ptr2ProxyNode(nullptr);
To me (and I am a newbie to C++) that initialization looks like a function call as well. Is that incorrect ?

Answer 1

On this line:
test1.flane_pointers(nullptr);
You're trying to call flane_pointers as though it were a member function. shared_ptr can't be called like a function, so you get the compiler error.
If you want to initialize flane_pointers, you can just assign to it:
test1.flane_pointers = nullptr;
Or alternatively, you could do the assignment when you create test1:
// Initialize test1 with a nullptr
_test_ test1{nullptr};

Answer 2

If your intent is to initialize flane_pointers to nullptr, you should use initialization of the below form:
shared_ptr<shared_ptr<ProxyNode>> flane_pointers = nullptr;
in struct _test_
or
test1.flane_pointers = nullptr;
in main.
The other form of initialization you are trying to do is interpreted as a function call in main and as a function declaration in struct _test_.
In the linked post,
std::shared_ptr<std::shared_ptr<ProxyNode> > ptr2ptr2ProxyNode(nullptr);
is in main and can only be interpreted as a variable declaration and not a function call because it does not have a function call syntax as the variable is preceded by the type std::shared_ptr >.
To avoid confusion, it is better (from C++11 onwards) to declare and initialize variables with the brace-enclosed initializer {}.

Answer 3

The line
test1.flane_pointers(nullptr);
is treated a function call. That's the source of the error. Use assignment instead.
test1.flane_pointers = nullptr;
And
shared_ptr<shared_ptr<ProxyNode>> flane_pointers(nullptr);
is not a valid form of in-member initialization. You may use
shared_ptr<shared_ptr<ProxyNode>> flane_pointers{nullptr};
or
shared_ptr<shared_ptr<ProxyNode>> flane_pointers = nullptr;