[英]Is there a way to implement covariant return types when using a vector of member functions in C++?
My base class will look something like (with constructors of course): 我的基类看起来像(当然有构造函数):
class gBase
{
public:
// The user will implement a vector of a vector of functions that can be called as g[i][alpha](k)
virtual vector<cdouble (gBase::*)(double)> operator[] (uint i) = 0;
};
and I want a possible implementation to look something like this: 我希望一个可能的实现看起来像这样:
class g : gBase
{
public:
g() : g_funcs({{g_00, g_01}}) {}
vector<cdouble (g::*)(double)> operator[] (uint i)
{
return g_funcs[i];
}
private:
vector<vector<cdouble (g::*)(double)> > g_funcs;
// define each function.
cdouble g_00(double k)
{
return 5.0;
}
cdouble g_01(double k)
{
return 3.0;
}
};
Where am I going wrong in defining g_funcs? 在定义g_funcs时我哪里出错了? I run into this:
我遇到了这个:
return type is not identical to nor covariant with return type "std::__1::vector<cdouble (gBase::*)(double), std::__1::allocator<cdouble (gBase::*)(double)>>" of overridden virtual function "gBase::operator[]"
A std::vector<T>
and std::vector<U>
are not covariant even if T
and U
are covariant. 即使
T
和U
是协变的, std::vector<T>
和std::vector<U>
也不协变。 With template types, each specialization is it's own unique type with no relation to the other besides the template name. 对于模板类型,每个专门化都是它自己的唯一类型,除了模板名称之外,其他类型都没有关系。
What you need is a vector of a common type, and you can get that using std::function
. 您需要的是通用类型的向量,您可以使用
std::function
获得它。 If both functions return a std::vector<std::function<double(double)>>
then the derived function will override the base one. 如果两个函数都返回一个
std::vector<std::function<double(double)>>
则派生函数将覆盖基函数。 You could then populate the functions in the vector by using a lambda that captures this
so it has the object to call the member function on. 然后,您可以使用捕获了
this
的lambda来填充矢量中的函数,以便它具有对象来调用成员函数。
If you can't do that another option would be to use a std::vector<std::function<double(gbase const*, double)>>
and then you would need pass a pointer to the object you want to call the function on plus the parameter. 如果您不能执行此操作,则另一种选择是使用
std::vector<std::function<double(gbase const*, double)>>
,然后需要将指针传递给您要调用的对象功能加上参数。
You will have to return a std::vector<cdouble (gBase::*)(double)>
, because there is no relation between std::vector<cdouble (gBase::*)(double)>
and std::vector<cdouble (g::*)(double)>
您将必须返回
std::vector<cdouble (gBase::*)(double)>
,因为std::vector<cdouble (gBase::*)(double)>
与std::vector<cdouble (g::*)(double)>
之间没有关系std::vector<cdouble (g::*)(double)>
Note also that g[i][alpha](k)
isn't going to work to call one of those functions, as you aren't passing a g
(as a gBase
) that will be this
. 还要注意,
g[i][alpha](k)
不会调用这些函数之一,因为您没有传递将是this
的g
(作为gBase
)。 You could instead 你可以代替
(g.*g[i][alpha])(k)
Or with C++17 或使用C ++ 17
std::invoke(g[i][alpha], g, k);
But it really sounds like you want to bundle up this
with the functions in the vector. 但它确实听起来像你要束起来
this
与载体的功能。 In that case you should have 在这种情况下,您应该
class gBase
{
public:
// The user will implement a vector of a vector of functions that can be called as g[i][alpha](k)
virtual std::vector<std::function<double(double)> > operator[] (uint i) = 0;
};
class g : public gBase
{
public:
g() : g_funcs({{[this](double k){ return g_00(k); }, [this](double k){ return g_01(k); }}}) {}
std::vector<std::function<double(double)> > operator[] (uint i)
{
return g_funcs[i];
}
private:
std::vector<std::vector<std::function<double(double)> > > g_funcs;
// define each function.
cdouble g_00(double k)
{
return 5.0;
}
cdouble g_01(double k)
{
return 3.0;
}
};
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