The Shared_ptr Class（memory header）

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Initialization

1.A default initialized smart pointer holds a null pointer.

shared_ptr<vector<string>> p;  // shared_ptr that can point at a vector of string.

2.Using make_shared function allocates and initializes an object in dynamic memeory and returns a shared ptr hat points to that object.

auto p = make_shared<vector<string>>(10,'9');  // P points to a string with value 9999999999.

3.Initializing a smart ptr from a pointer returned by new.

• must use direct initialization.
• can't implicitly convert a plain ptr to a smart ptr.

shared_ptr<int> p1 = new int(1024);  // error:must use direct initialization.(not copy initialization)
shared_ptr<int> p2(new int(1024));  // ok:uses direct initialization.

shared_ptr<int> clone(int p){
    return new in(p);    // error:implicit conversion to shared_ptr<int>.
}
shared_ptr<int> clone(int p){
    return shared_ptr<int>(new in(p));    // ok:explicitly create a shared_ptr<int> from int*.
}

Copying and Assigning

1.We can think of a shared_ptr as if it has an associated counter, usually referred to as a reference count.

Counter incremented when:

• use it(shared_ptr) as the right-hand operand of an assignment;
• pass it to or return it from a function by value.

Once a shared ptr's counter goes to zero, the shared_ptr automatically frees the object that it manages.
Memory is not freed until the last shared_ptr goes away.

Classes with resources that have dynamic lifetime

1.Each vector "owns" its own elememts. When we copy a vector, the elements in the original vector and in the copy are separate from one another.

vector<string> v1;
{  // new scope.
    vector<string> v2 = {"a", "an", "the"};
    v1 = v2;  // copies the elements from v2 into v1.
}  // v2 is destroyed, which destroys the elements in v2.
   // v1 has three elements, which are copies of the ones originally in v2.

• The elements allocated by a vector exist only while the vector itself exists.

2.Use dynamic memory to allow multiple objects to share the same state(data).

• To ensure that the elements continue to exist, we'll store the vector in dynamic memory.

#ifndef MY_STRBLOB_H
#define MY_STRBLOB_H
#include <vector>
#include <string>
#include <initializer_list>
#include <memory>
#include <stdexcept>

using namespace std;

class StrBlob {
public:
	typedef vector<string>::size_type size_type;
	StrBlob();
	StrBlob(initializer_list<string> il);  // initializer_list is a library type that represents
	                                       // an array of values of the specified type.
	size_type size() const { return data->size(); }
	bool empty() const { return data->empty(); }

	void push_back(const string& t) { data->push_back(t); }
	void pop_back();

	string& front();
	string& back();

private:
	shared_ptr<vector<string>> data;
	void check(size_type i, const string& msg) const;
};

StrBlob::StrBlob(): data(make_shared<vector<string>>()){}
StrBlob::StrBlob(initializer_list<string> il):
	data(make_shared<vector<string>>(il)){}

void StrBlob::check(size_type i, const string& msg) const {
	if (i >= data->size())
		throw out_of_range(msg);
}

string& StrBlob::front() {
	check(0, "front on empty StrBlob");
	return data->front();
}

string& StrBlob::back() {
	check(0, "back on empty StrBlob");
	return data->back();
}

void StrBlob::pop_back() {
	check(0, "pop_back on empty StrBlob");
	data->pop_back();
}
#endif // !MY_STRBLOB_H

#include <iostream>
#include "Strblob.h"

int main() {
	StrBlob b1;
	{
		StrBlob b2 = { "a","an","the" };
		b1 = b2;
		b2.push_back("about");
		cout << b2.size() << endl;
	}
	cout << b1.size() << endl;
	cout << b1.front() << " " << b1.back() << endl;

	StrBlob b3 = b1;
	cout << b3.front() << " " << b3.back() << endl;

	return 0;
}
// output:
// 4
// 4
// a about
// a about

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