Question

I came up with my own little class called TinyVector. And now I’m trying to use std::inner_product on it. But I can’t get it to work and I don’t understand why this does not work.
I’m using Visual Studio 2012.

#include <iostream>
#include <vector>
#include <numeric>

using namespace std;

template<class T, int N>
class TinyVector {
public:
    TinyVector() { data = vector<T>(N, 0); }
    explicit TinyVector(const T initVal) { data = vector<T>(N, initVal); }
    ~TinyVector() { data.clear(); }
    inline T& operator[](int i) { return data[i]; }
    inline T operator[](int i) const { return data[i]; }
    inline vector<T>::const_iterator begin() const { return data.begin(); } //Line 15
    inline vector<T>::const_iterator end() const { return data.end(); } //Line 16
private:
    vector<T> data;
};

template<class T, int N>
inline double dot(const TinyVector<T,N>& A, const TinyVector<T,N>&  B)
{
    return inner_product(A.begin(), A.end(), B.begin(), 0.0);
}

int main()
{
    TinyVector<double, 10> Ty;
    for (int i = 0; i < 10; ++i)
        Ty[i] = i;

    cout << dot(Ty,Ty) << endl;
}

The compiler tells me:
syntax error : missing ‘;’ before identifier ‘begin’ on line 15.
missing type specifier – int assumed. Note: C++ does not support default-int on line 15.
syntax error : missing ‘;’ before identifier ‘end’ on line 16.
missing type specifier – int assumed. Note: C++ does not support default-int on line 16.

But changing vector<T>::const_iterator into vector::const_iterator doesn’t seem like the way to go.
Also changing it to ‘auto’ does not work. This gives me “expected a trailing return type”.
If I delete line 15,16 and 17 and replace A.begin() with A.data.begin() and the next two arguments accoringly, everything is fine.
But why doesn’t my original code work, and how can I get it to work?

Answer 1

You need to write

inline typename vector<T>::const_iterator begin() const { return data.begin(); } //Line 15
       ^^^^^^^^

This is because vector<T>::const_iterator is a dependent name (it is dependent on the type parameter T) and so the compiler needs to be told that vector<T>::const_iterator is a type (and not, say, an enum value or static data member).