标签:memory back header vector Shared shared data ptr StrBlob
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.
StrBlob.h
#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
StrBlob.cpp
#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
标签:memory,back,header,vector,Shared,shared,data,ptr,StrBlob 来源: https://www.cnblogs.com/Dy2MoRaw/p/15873049.html
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