CODEWITHSUNDEEP
c++vectoriteratormemcpy
ratzip
ratzip

Reputation: 113

how to memcpy iterator element in C++?

I have the following code in C++:

typedef struct 
{
   int age;
   int roomNumber;
} Student;

Student student;
student.age = 20;
student.roomNumber = 10;

vector<Student> studentV;
student.push_back(student);
student.push_back(student);
student.push_back(student);

Student student1[3];

int i;

for(vector<Student>::const_iterator iterator = studentV.begin();
    iterator != studentV.end(); ++iterator,++i)
{
   memcpy(&student1[i], iterator, sizeof(Student));
}

It show the following message for the memcpy part:

error: cannot convert 'std::vector<Student>::const_iterator {aka __gnu_cxx::__normal_iterator<const Student*, std::vector<Student> >}' to 'const void*' for argument '2' to 'void* memcpy(void*, const void*, size_t)'

What is the problem and how to fix it? is iterator can not be copied like this?

Upvotes: 4

Views: 4388

Answers (4)

Matthieu Poullet
Matthieu Poullet

Reputation: 427

You should take advantage of the actual C++11 facilities and avoid using memcpy at all:

#include <iostream>
#include <vector>
#include <array>

typedef struct
{
    int age;
    int roomNumber;
} Student;

int main() {
    Student student{ 20, 10 };
    std::vector<Student> studentV{ student, student, student };
    std::array<Student, 3> student1;

    if (studentV.size() <= student1.size())
        std::copy(studentV.begin(), studentV.end(), student1.begin());
    for (const auto& s : student1) {
        std::cout << s.age << " " << s.roomNumber << std::endl;
    }
}

Upvotes: 3

Tomek
Tomek

Reputation: 4659

Use std::copy algorithm. It will use plain memory copy routines (like memcpy) if the object is POD or will use copy constructor otherwise. If will also protect you from copying non-copyable objects (by printing an error during compilation), something your code may do and which may cause hard to trace errors (like double free inside copied non-copyable objects).

Upvotes: 1

neuro
neuro

Reputation: 15180

What you want to do is copy what the iterator refers to. Use *iterator to get the object referenced by the iterator and & to get its address. By the way DO NOT use memcpy but the copy constructor:

student1[i] = *iterator;

Upvotes: 4

Wojtek Surowka
Wojtek Surowka

Reputation: 21003

The second argument of memcpy should be address of Student object, so you should use &*iterator instead of iterator. Because of operator overloading those two are not equivalent. But I would recommend not to use memcpy at all.

Upvotes: 5

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