Segmentation fault about c++ smart pointers? - c++

Question

Hello I'm a fresh man to c++. And today when I test a project of my code, I encountered a problem that made me feel confused.
I want to use smart pointer in my project of parse JSON, so I pass a line of string to the class: json_content, and I want the member of json_content, json_value to get the string. The compiler didn't give me any warning or error, but when I run the a.out file, it tells me that segmentation fault. I searched a lot in Google, however I didn't find any solutions to this problem. Could any one help me? Thanks a lot! :)
BTW, my OS is MacOSX x86_64-apple-darwin18.2.0, compiler is Apple LLVM version 10.0.0 (clang-1000.10.44.4)
Here is the code:
#include <string>
#include <iostream>
#include <memory>
#include <typeinfo>
using namespace std;
class json_content {
public:
string json_value;
};
int main()
{
shared_ptr<json_content> c;
shared_ptr<string> p2(new string("this is good"));
// segmentation fault
c->json_value = *p2;
// this is also bad line!
c->json_value = "not good, too!";
return 0;
}

Answer 1

By default, a shared_ptr is nullptr (see API). You can't de-reference a nullptr. You need to initialize c first:
#include <string>
#include <iostream>
#include <memory>
#include <typeinfo>
using namespace std;
class JsonContent {
public:
string json_value;
};
int main() {
shared_ptr<JsonContent> c = std::make_shared<JsonContent>();
shared_ptr<string> p2 = std::make_shared<string>("This is good.");
c->json_value = *p2;
c->json_value = "This is also good!";
cout << c->json_value << endl;
return 0;
}
Demo: http://cpp.sh/5fps7n.