not able to create const object of class derived from std::vector
I am deriving stl container class vector and trying to overload the [] operators to provide range checking on accessing element with subscript. But I get compile errors when try to create const object of my derived class Vec .
For example below is the code:
template<class T>
class Vec : public vector<T>
{
public:
Vec(): vector<T>()
{
}
Vec(int i):vector<T>(i)
{
}
T& operator[](int i)
{
return at(i);
}
const T& operator[](int i) const
{
if(i>=size())
cout<<"error"<<endl;
return at(i);
}
};
void main()
{
Vec<int> v1(10); //works fine
const Vec<int> v(10); //error
}
Why the code const vector v(10); works but const Vec v1(10); doesn't work. Is there something I am missing? Why am I not able to create const object?
The STL classes like std::vector are generally not meant to be derived from.
But the code works correctly like this:
template<class T>
class Vec : public vector<T>
{
public:
Vec(): vector<T>()
{
}
Vec(int i):vector<T>(i)
{
}
T& operator[](int i)
{
return vector<T>::at(i);
}
const T& operator[](int i) const
{
if(i>=vector<T>::size())
cout<<"error"<<endl;
return vector<T>::at(i);
}
};
int main()
{
Vec<int> v1(10); //works fine
const Vec<int> v(10); //error
v1.push_back(5);
v1.push_back(6);
int i = v1[3];
}
The functions at and size from the base std::vector need to be called with using either vector<T>::at(i) or this->at(i) . This is always true when the base class is a template: Then " vector<T> " is a dependent name (its meaning depends on the template argument T ), and in that case its members are not automatically exported to the scope of the Vec class definition.
Also cout<"error" must be cout<<"error" , and main must return an int .
const does not pose a problem in this case.
In general, push_back cannot be used on a const std::vector , because it modifies the vector. But copy initialization or list initialization can be used to create a non-empty const std::vector . For example
const std::vector<int> vec {1, 2, 3};
Bjarne Stroustrup covers this issue in "A Tour of C++".
template<typename T>
class Vec : public std::vector<T> {
public:
using vector<T>::vector; // use the constructors from vector (under the name Vec)
T& operator[](int i)
{ return vector<T>::at(i); }
const T& operator[](int i) const
{ return vector<T>::at(i); }
}
"Vec inherits everything from vector except for the subscript operations that it redefines to do range checking."
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