First of all, shared_ptr can be a nullptr. Nothing wrong passing a null shared pointer.
In fact, a null shared pointer is an actual Null Object, which means that it’s designed exactly for those kinds of uses.
but if we want to pass the share_ptr as default nullptr reference, we need to put a const before that
Example is better than words, here is an example how to pass shared
#include <memory>
#include <iostream>
class A
{
public:
// OK, for reference we need const
A( const std::shared_ptr<std::string>& t = nullptr ) : my_shared(t){}
// ERROR , will have compile error
// A( std::shared_ptr<std::string>& t = nullptr ) : my_shared(t){}
// pass by value works without const
// A( std::shared_ptr<std::string> t = nullptr ) : my_shared(t){ }
void output() {
if(my_shared) {
std::cout << my_shared->c_str() << std::endl;
} else {
std::cout <<” null prt” << std::endl;
}
}
private:
std::shared_ptr<std::string> my_shared;
};
int main()
{
std::shared_ptr<std::string> t = nullptr;
A a( t );
a.output();
A a2( nullptr );
a2.output();
std::shared_ptr<std::string> t2 = std::make_shared<std::string>(“test shared”);
A b(t2);
b.output();
}
C++ said we can bind the value of a temporary expression to a constant reference,
and the lifetime of the expression is extended to that of the reference.
But we cannot do this with a non-constant (lvalue) reference.
string& s3 = string("s3"); // ERROR! non-const reference not allowed!
References
https://stackoverflow.com/questions/14612933/const-reference-default-value
https://stackoverflow.com/questions/25920681/what-is-the-difference-between-an-empty-and-a-null-stdshared-ptr-in-c
