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?