declaring a vector as a class member - c++

Question

I have simple class in a header file: a.hh
#ifndef a_hh
#define a_hh
class a
{
public:
int i;
a()
{
i = 0;
}
};
#endif
Then i have a file:b.cc
#include <iostream>
#include "a.hh"
using namespace std;
int main(int argc, char** argv)
{
a obj;
obj.i = 10;
cout << obj.i << endl;
return 0;
}
>
Till this point everything is fine.
I compile the code and it compiles fine.
But as soon as i add a vector in the class:
#ifndef a_hh
#define a_hh
class a
{
public:
int i;
vector < int > x;
a()
{
i = 0;
}
};
#endif
I get a compilation error as below:
> CC b.cc
"a.hh", line 7: Error: A class template name was expected instead of vector.
1 Error(s) detected.
What is the problem with declaring a vector here as a member?

Answer 1

You need to #include <vector> and use the qualified name std::vector<int> x;:
#ifndef a_hh
#define a_hh
#include <vector>
class a{
public:
int i;
std::vector<int> x;
a() // or using initializer list: a() : i(0) {}
{
i=0;
}
};
#endif
Other points:
(as commented by EitanT) I removed the additional qualification a:: on the constructor
have a read of Why is "using namespace std" considered bad practice?

Answer 2

declaring a vector as a class member:
#include <iostream>
#include <vector>
using namespace std;
class class_object
{
public:
class_object() : vector_class_member() {};
void class_object::add_element(int a)
{
vector_class_member.push_back(a);
}
void class_object::get_element()
{
for(int x=0; x<vector_class_member.size(); x++)
{
cout<<vector_class_member[x]<<" \n";
};
cout<<" \n";
}
private:
vector<int> vector_class_member;
vector<int>::iterator Iter;
};
int main()
{
class_object class_object_instance;
class_object_instance.add_element(3);
class_object_instance.add_element(6);
class_object_instance.add_element(9);
class_object_instance.get_element();
return 0;
}

Answer 3

1.You need to #include <vector> and using namespace std, then a.hh just like below:
#ifndef a_hh
#define a_hh
#include <vector>
using namespace std;
class a
{
public:
int i;
vector <int> x;
a()
{
i = 0;
}
};
#endif
2. If you don't want to only use std namespace in all your code, you can specified the namespace before type, just like std::vector<int> x;