C++ - 错误：未在此范围内声明'function'

我有以下问题:错误:未在此范围内声明'kleiner'我的教授告诉我，我的代码对他来说工作得很好。这些目录都包含在构建选项中(我使用Code::Blocks)。谁能告诉我是什么问题吗?

main.cpp

#include <iostream>
#include "vector.h"
using namespace std;
int main(int argc, char *argv[])
{
    Vector v1;
    cout << "v1: " << v1 << endl;
    Vector v2(8);
    cout << "v2: " << v2 << endl;
    cout << "Minimum von v2: " << v2.min() << endl;
    Vector v3(v2);
    cout << "v3: " << v3 << endl;
    cout << "Anzahl von v3: " << v3.getAnzahl() << endl;
    if ( kleiner( v3[2], v2[5] ) )//<<--<<--<<-- HERE IS THE PROBLEM
        cout << v3[2] << " ist kleiner als " << v2[5] << endl;
    int arr[5] = { 10, 5, 2, 3, 12 };
    Vector v4;
    cout << "v4: " << v4 << endl;
    v4.setVector( arr, 4 );
    cout << "v4 nach set: " << v4 << endl;
    cout << "Minimum von v4: " << v4.min() << endl;
    cout << "Anzahl von v4: " << v4.getAnzahl() << endl;
    return 0;
}

vector.h

#ifndef VECTOR_H
#define VECTOR_H
#include <iostream>
using namespace std;
class Vector
{
      int* v;
      int anzahl;
public:
       Vector(int anzahl = 10);
       Vector( const Vector& vec ); // Kopierkonstruktor
       ~Vector();
       friend bool kleiner( const int& a, const int& b );
       int min() const;
       int getAnzahl() const;
       int operator[]( const int i ) const;
       void setVector( int* sv, int sanzahl);
       friend ostream& operator<< ( ostream& os, const Vector& v );
};
#endif

vector.cpp

#include "vector.h"
Vector::Vector( int a ) : anzahl(a)
{
    v = new int[a];
    for ( int i = 0; i < a; i++ )
        v[i] = i;
}
Vector::Vector( const Vector& vec )
{
    anzahl = vec.getAnzahl();
    v = new int[anzahl];
    for ( int i = 0; i < anzahl; i++ )
        v[i] = vec[i];
}
Vector::~Vector()
{
    delete[] v;
    v = NULL;
}
bool kleiner( const int& a, const int& b )
{
     return ( a < b );
}
int Vector::min() const
{
     int min = v[0];
     for ( int i = 1; i < anzahl; i++ )
     {
         if ( v[i] < min )
             min = v[i];
     }
     return min;
}
int Vector::getAnzahl() const
{
    return anzahl;
}
int Vector::operator[] ( const int i ) const
{
    return v[i];
}
void Vector::setVector( int* sv, int sanzahl )
{
     delete[] v; // alten Inhalt loeschen
     anzahl = sanzahl;
     v = new int[anzahl];
     for ( int i = 0; i < anzahl; i++ )
     v[i] = sv[i];
     return;
}
ostream& operator<< ( ostream& os, const Vector& v )
{
     for ( int i = 0; i < v.anzahl; i++ )
         os << v[i] << ", ";
     return os;
}