在 C++ 中实现惰性赋值时出错
Error in implementing lazy assign in C++
我在实现 https://eigen.tuxfamily.org/dox/TopicInsideEigenExample.html 中的惰性分配和添加时遇到了一些奇怪的问题。
密码是
template<typename Derived> class Base;
template<typename Derived, typename OtherDerived> class SumOp;
template<typename Derived, typename OtherDerived>
class SumOp: public Base<SumOp<Derived, OtherDerived>>{
public:
Derived & lhs;
OtherDerived & rhs;
SumOp(Derived & lhs_, OtherDerived & rhs_):lhs(lhs_), rhs(rhs_){}
double packet(size_t index, Base<OtherDerived>& src){
return lhs.packet(index)+ rhs.packet(index);
}
};
template<typename Derived, typename OtherDerived>
struct Assign{
static Derived& run(Derived & dst, OtherDerived & src){
size_t length = dst.size();
for (size_t index =0; index < length; index++){
dst.copyPacket(index, src);
}
return dst;
}
};
template<typename Derived>
class Base{
public:
Base(){}
Derived& derived(){
return *static_cast<Derived*>(this);
}
const Derived& derived() const{
return *static_cast<const Derived*>(this);
}
template<typename OtherDerived>
SumOp<Derived,OtherDerived> operator+(Base<OtherDerived> & other){
return SumOp<Derived, OtherDerived>(this->derived(), other.derived());
}
template<typename OtherDerived>
Derived & operator=(Base<OtherDerived>& other){
return Assign<Derived, OtherDerived>::run(derived(), other.derived());
}
template<typename OtherDerived>
void copyPacket(size_t index, Base<OtherDerived> & other){
derived().writePacket(index, other.derived().packet(index));
}
};
class Vector: public Base<Vector> {
public:
double * data;
size_t nRow;
Vector(size_t nRow_):nRow(nRow_){
data = (double *)malloc(sizeof(double)*nRow);
}
~Vector(){
free(data);
}
template<typename OtherDerived>
Vector& operator=( Base<OtherDerived>& other){
return Base<Vector>::operator=(other);
}
size_t size(){
return nRow;
}
void writePacket(size_t index, double src){
data[index] = src;
}
double packet(size_t index){
return data[index];
}
};
简而言之,问题在于当我在 Vector class 中调用 operator=
时,编译器抱怨我将左值 Base<SumOp<Vector, Vector>>
传递给运算符而不是 SumOp<Vector, Vector>&
。后者是在调用 operator+
时创建的,其中 returns SumOp<Vector, Vector>
。
编译器错误是
no known conversion for argument 1 from ‘SumOp<Vector, Vector>’ to ‘Base<SumOp<Vector, Vector> >&’
我不确定为什么 Eigen 中的实现没问题以及如何解决这个问题。
感谢您的帮助。
编辑
调用以下函数时出现问题
void test_vector(){
Vector a(10), b(10), c(10);
for (int i=0;i<10;i++){
a.data[i]=1.0;
b.data[i]=1.0;
}
//This is very it goes wrong
c = a+b;
for (int i =0 ; i<10;i++){
std::cout << c.data[i] << std::endl;
}
};
对于 g++-6,错误显示为
invalid initialization of non-const reference of type ‘Base<SumOp<Vector, Vector> >&’ from an rvalue of type ‘Base<SumOp<Vector, Vector> >’
c = a+b;
~^~
In file included from /home/ran/Desktop/experiment/PointerMatrix/vector.cpp:6:0:
/home/ran/Desktop/experiment/PointerMatrix/vector.h:76:21: note: initializing argument 1 of ‘Vector& Vector::operator=(Base<OtherDerived>&) [with OtherDerived = SumOp<Vector, Vector>]’
Vector& operator=( Base<OtherDerived>& other){
问题是因为 operator
函数的参数不是常量引用。 operator+
returns 临时 SumOp
对象,不能作为非常量引用传递。 (一些编译器可能支持这个作为扩展。)
将参数更改为 const &
类型应该可以解决此问题。
我在实现 https://eigen.tuxfamily.org/dox/TopicInsideEigenExample.html 中的惰性分配和添加时遇到了一些奇怪的问题。
密码是
template<typename Derived> class Base;
template<typename Derived, typename OtherDerived> class SumOp;
template<typename Derived, typename OtherDerived>
class SumOp: public Base<SumOp<Derived, OtherDerived>>{
public:
Derived & lhs;
OtherDerived & rhs;
SumOp(Derived & lhs_, OtherDerived & rhs_):lhs(lhs_), rhs(rhs_){}
double packet(size_t index, Base<OtherDerived>& src){
return lhs.packet(index)+ rhs.packet(index);
}
};
template<typename Derived, typename OtherDerived>
struct Assign{
static Derived& run(Derived & dst, OtherDerived & src){
size_t length = dst.size();
for (size_t index =0; index < length; index++){
dst.copyPacket(index, src);
}
return dst;
}
};
template<typename Derived>
class Base{
public:
Base(){}
Derived& derived(){
return *static_cast<Derived*>(this);
}
const Derived& derived() const{
return *static_cast<const Derived*>(this);
}
template<typename OtherDerived>
SumOp<Derived,OtherDerived> operator+(Base<OtherDerived> & other){
return SumOp<Derived, OtherDerived>(this->derived(), other.derived());
}
template<typename OtherDerived>
Derived & operator=(Base<OtherDerived>& other){
return Assign<Derived, OtherDerived>::run(derived(), other.derived());
}
template<typename OtherDerived>
void copyPacket(size_t index, Base<OtherDerived> & other){
derived().writePacket(index, other.derived().packet(index));
}
};
class Vector: public Base<Vector> {
public:
double * data;
size_t nRow;
Vector(size_t nRow_):nRow(nRow_){
data = (double *)malloc(sizeof(double)*nRow);
}
~Vector(){
free(data);
}
template<typename OtherDerived>
Vector& operator=( Base<OtherDerived>& other){
return Base<Vector>::operator=(other);
}
size_t size(){
return nRow;
}
void writePacket(size_t index, double src){
data[index] = src;
}
double packet(size_t index){
return data[index];
}
};
简而言之,问题在于当我在 Vector class 中调用 operator=
时,编译器抱怨我将左值 Base<SumOp<Vector, Vector>>
传递给运算符而不是 SumOp<Vector, Vector>&
。后者是在调用 operator+
时创建的,其中 returns SumOp<Vector, Vector>
。
编译器错误是
no known conversion for argument 1 from ‘SumOp<Vector, Vector>’ to ‘Base<SumOp<Vector, Vector> >&’
我不确定为什么 Eigen 中的实现没问题以及如何解决这个问题。
感谢您的帮助。
编辑
调用以下函数时出现问题
void test_vector(){
Vector a(10), b(10), c(10);
for (int i=0;i<10;i++){
a.data[i]=1.0;
b.data[i]=1.0;
}
//This is very it goes wrong
c = a+b;
for (int i =0 ; i<10;i++){
std::cout << c.data[i] << std::endl;
}
};
对于 g++-6,错误显示为
invalid initialization of non-const reference of type ‘Base<SumOp<Vector, Vector> >&’ from an rvalue of type ‘Base<SumOp<Vector, Vector> >’
c = a+b;
~^~
In file included from /home/ran/Desktop/experiment/PointerMatrix/vector.cpp:6:0:
/home/ran/Desktop/experiment/PointerMatrix/vector.h:76:21: note: initializing argument 1 of ‘Vector& Vector::operator=(Base<OtherDerived>&) [with OtherDerived = SumOp<Vector, Vector>]’
Vector& operator=( Base<OtherDerived>& other){
问题是因为 operator
函数的参数不是常量引用。 operator+
returns 临时 SumOp
对象,不能作为非常量引用传递。 (一些编译器可能支持这个作为扩展。)
将参数更改为 const &
类型应该可以解决此问题。