指定一些模板参数(Specifying some template parameters)

 我想编写一个自己的Vector类和一个计算交叉乘积的函数。 
 
 我的Vector-class有 
 
template<class T, int dim, bool isRowVector>
 
 T是条目的类型，dim是维度，isRowVector指定向量是行向量还是列向量。 
 
 我重载了一些Vector的运算符，特别是<<： 
 
template <class T, int dim, bool isRowVec>
std::ostream& operator<<(std::ostream& os, Vector<T, dim, isRowVec>& vec) // <<
{
    os << "[" << vec[0];

    for(typename std::vector<T>::iterator it = vec.begin()+1; it != vec.end(); it++) {
        os << "," << *it;
    }

    os << "]";

    if(!isRowVec) { os << "^T"; }

    return os;
}
 
 现在函数crossProduct应该只适用于dim = 3的Vectors。所以我指定了这样的函数： 
 
template <class T, bool isRowVec>
Vector<T, 3, isRowVec> crossProduct (Vector<T, 3, isRowVec> a, Vector<T, 3, isRowVec> b) {
    T arr[3] = { a[1]*b[2]-a[2]*b[1], a[2]*b[0]-a[0]*b[2], a[0]*b[1]-a[1]*b[0] };
    Vector<T, 3, isRowVec> newVec(arr);
    return newVec;
}
 
 省略函数模板中的dim参数，因为它不必被猜测或声明。 
 
 这似乎不是一个问题，因为 
 
Vector<float,3,true> a = crossProduct(f,g);
 
 不会产生编译错误，其中f和g都是 
 
Vector<float,3,true>
 
 但是当我试着打电话的时候 
 
std::cout << crossProduct(f, g) << std::endl;
 
 我明白了 
 
error: no match for 'operator<<' (operand types are 'std::ostream {aka std::basic_ostream<char>}' and 'Vector<float, 3, true>')
  std::cout << crossProduct(f, g) << std::endl;
            ^
 
 有任何想法吗？ 

I want to write an own Vector-class and a function which calculates the cross product.
 
My Vector-class has the
 
template<class T, int dim, bool isRowVector>
 
T is the type of the entries, dim the dimension and isRowVector specifies whether the vector is a row or a column vector.
 
I overloaded some operators of Vector, particularly <<:
 
template <class T, int dim, bool isRowVec>
std::ostream& operator<<(std::ostream& os, Vector<T, dim, isRowVec>& vec) // <<
{
    os << "[" << vec[0];

    for(typename std::vector<T>::iterator it = vec.begin()+1; it != vec.end(); it++) {
        os << "," << *it;
    }

    os << "]";

    if(!isRowVec) { os << "^T"; }

    return os;
}
 
Now the function crossProduct should only apply on Vectors with dim = 3. So I specified the function like this:
 
template <class T, bool isRowVec>
Vector<T, 3, isRowVec> crossProduct (Vector<T, 3, isRowVec> a, Vector<T, 3, isRowVec> b) {
    T arr[3] = { a[1]*b[2]-a[2]*b[1], a[2]*b[0]-a[0]*b[2], a[0]*b[1]-a[1]*b[0] };
    Vector<T, 3, isRowVec> newVec(arr);
    return newVec;
}
 
omitting the dim parameter in the function template, since it does not have to be guessed or stated.
 
This doesn't seem to be a problem, since
 
Vector<float,3,true> a = crossProduct(f,g);
 
does not create a compile-error, where f and g are both
 
Vector<float,3,true>
 
But when I try to call
 
std::cout << crossProduct(f, g) << std::endl;
 
I get
 
error: no match for 'operator<<' (operand types are 'std::ostream {aka std::basic_ostream<char>}' and 'Vector<float, 3, true>')
  std::cout << crossProduct(f, g) << std::endl;
            ^
 
Any ideas?

原文：https://stackoverflow.com/questions/27076490