[英]Why is it not allowed to specialize a member function template within a class?
When I try to specialize a public member function template within the class definition/declaration: 当我尝试在类定义/声明中专门化公共成员函数模板时:
#include <iostream>
class surfaceMesh
{
public:
// Uncomment for Version 0 and 1
class AREA_AVERAGE {};
class ANGLE_AVERAGE {};
template<class Average>
void vertexNormals() {}
// Uncomment: Version 0
//template<>
//void vertexNormals<AREA_AVERAGE> ()
//{
//std::cout << "AREA_AVERAGE" << std::endl;
//}
//template<>
//void vertexNormals<ANGLE_AVERAGE> ()
//{
//std::cout << "ANGLE_AVERAGE" << std::endl;
//}
};
// Uncommend for version 1
template<>
void surfaceMesh::vertexNormals<surfaceMesh::AREA_AVERAGE> ()
{
std::cout << "AREA_AVERAGE" << std::endl;
};
template<>
void surfaceMesh::vertexNormals<surfaceMesh::ANGLE_AVERAGE> ()
{
std::cout << "ANGLE_AVERAGE" << std::endl;
};
int main()
{
surfaceMesh m;
m.vertexNormals<surfaceMesh::AREA_AVERAGE>();
m.vertexNormals<surfaceMesh::ANGLE_AVERAGE>();
return 0;
}
For Version 0, the error is: 对于版本0,错误是:
main.cpp:19: error: template-id ‘vertexNormals<mesh::AREA_AVERAGE>’ in declaration of primary template
main.cpp:24: error: explicit specialization in non-namespace scope ‘class mesh’
main.cpp:25: error: template-id ‘vertexNormals<mesh::ANGLE_AVERAGE>’ in declaration of primary template
main.cpp:25: error: ‘void mesh::vertexNormals()’ cannot be overloaded
main.cpp:19: error: with ‘void mesh::vertexNormals()’
Version 1 compiles and runs. 版本1编译并运行。 Of course, usually I am separating the class declaration and definition, but I would really like to know why this happens.
当然,通常我将类的声明和定义分开,但是我真的很想知道为什么会这样。
Also, is this a good way to specialize an interface? 另外,这是专门化接口的好方法吗? The other option would be to overload the function vertexNormals to take an object of AREA_AVERAGE or ANGLE_AVERAGE, but this is just a type telling me which kind of function I will be using, it is not supposed to be instantiated, so using templates 'feels' like a right choice.
另一个选择是重载功能vertexNormals以获取AREA_AVERAGE或ANGLE_AVERAGE的对象,但这只是一种类型,告诉我将使用哪种函数,不应实例化,因此使用模板“感觉”像一个正确的选择。
Why is it not allowed to specialize a member function template within a class?
为什么不允许在类中专门化成员函数模板?
Because that is the rule laid down by the C++ Standard. 因为那是C ++标准规定的规则。
As for what you want, a better approach is to use function overload rather than function specialization as: 至于您想要什么,更好的方法是使用函数重载而不是函数专门化:
class surfaceMesh
{
public:
// Uncomment for Version 0 and 1
class AREA_AVERAGE {};
class ANGLE_AVERAGE {};
template<class Average>
void vertexNormals()
{
//invoke the function overload
vertexNormals(static_cast<Average*>(0));
}
private:
//make the overloads private, so client will not call them!
void vertexNormals(AREA_AVERAGE *)
{
std::cout << "AREA_AVERAGE" << std::endl;
}
void vertexNormals(ANGLE_AVERAGE*)
{
std::cout << "ANGLE_AVERAGE " << std::endl;
}
};
The type of the expression static_cast<Average*>(0)
helps the compiler to choose the correct overload. 类型表达
static_cast<Average*>(0)
有助于编译器选择正确的过载。
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