See this example :
#include <iostream>
#include <memory>
class Foo {
public:
Foo() { std::cout << "Foo()\n"; }
~Foo() { std::cout << "~Foo()\n"; }
};
int main(){
auto deleter = [](Foo* p) {
if(!p) { std::cout << "Calling deleter on nullptr\n"; }
delete p;
};
std::shared_ptr<Foo> foo;
std::cout << "\nWith non-null Foo:\n";
foo = std::shared_ptr<Foo>(new Foo, deleter);
std::cout << "foo is " << (foo ? "not ":"") << "null\n";
std::cout << "use count=" << foo.use_count() << '\n';
foo.reset();
std::cout << "\nWith nullptr and deleter:\n";
foo = std::shared_ptr<Foo>(nullptr, deleter);
std::cout << "foo is " << (foo ? "not ":"") << "null\n";
std::cout << "use count=" << foo.use_count() << '\n';
foo.reset();
std::cout << "\nWith nullptr, without deleter:\n";
foo = std::shared_ptr<Foo>(nullptr);
std::cout << "foo is " << (foo ? "not ":"") << "null\n";
std::cout << "use count=" << foo.use_count() << '\n';
foo.reset();
}
The output is :
With non-null Foo:
Foo()
foo is not null
use count=1
~Foo()
With nullptr and deleter:
foo is null
use count=1
Calling deleter on nullptr
With nullptr, without deleter:
foo is null
use count=0
Here we see that shared_ptr
calls the contained deleter when it is initialized with nullptr
and a custom deleter.
It seems that, when initialized with a custom deleter, shared_ptr
considers it is "owning" nullptr and thus tries to delete it when it would delete any other owned pointer. Though it does not happen when no deleter is specified.
Is this intended behavior ? If so, what is the reason behind this behavior ?