[英]Templated Defaulted default constructor
I would like to have the definition of the default constructor be switched at compile time by a template parameter. 我想通过模板参数在编译时切换默认构造函数的定义。 I can get this to compile OK for a conversion constructor, but trying to use that method for a default constructor to be defaulted or not--useful if in the case of particular template parameter, the resulting class could be POD, but in another case, it could not--but I get a compiler error when doing so.
我可以让它为转换构造函数编译OK,但尝试使用该方法默认构造函数是否默认 - 如果在特定模板参数的情况下,生成的类可能是POD,但在另一种情况下,它不能 - 但这样做时我得到编译器错误。 Short of specializing the template and duplicating all the code that's the same, is there a way to do this?
如果没有专门化模板并复制所有相同的代码,有没有办法做到这一点? Here's a simplified version of what I was trying:
这是我尝试的简化版本:
#include<type_traits> // for enable_if
template <bool MyParameter>
class Demonstration
{
public:
//trivial copy, move constructors/assignment, and trivial destructor
constexpr Demonstration(Demonstration const &) = default;
constexpr Demonstration(Demonstration &&) = default;
Demonstration & operator= (Demonstration const &) = default;
Demonstration & operator= (Demonstration &&) = default;
~Demonstration() = default;
// this one gives "error: a template cannot be defauled"
template <bool Dummy=MyParameter, typename std::enable_if< Dummy , bool >::type=true >
Demonstration() = default;
// ok
template <bool Dummy=MyParameter, typename std::enable_if< !Dummy , bool >::type=false >
Demonstration() : myValue(0) {}
// ok
template <bool Dummy=MyParameter, typename std::enable_if< Dummy , bool >::type=true >
explicit constexpr Demonstration(unsigned char toConvert)
: myValue ( toConvert )
{
}
// ok
template <bool Dummy=MyParameter, typename std::enable_if< !Dummy , bool >::type=false >
explicit constexpr Demonstration(unsigned char toConvert)
: myValue ( toConvert > 100 ? 0 : toConvert )
{
}
// a lot of functions that do not depend on parameter go here
protected:
private:
unsigned char myValue;
};
GCC complains of your template: 海湾合作委员会抱怨你的模板:
error: a template cannot be defaulted
and Clang complains: 和Clang抱怨:
error: only special member functions may be defaulted.
That seems fair enough. 这看起来很公平。 A member function template is not a member function, let alone a special one.
成员函数模板不是成员函数,更不用说特殊函数了。
You would like Demonstration<bool P>
to be POD when P
is true and otherwise not necessarily so. 当
P
为真时,你希望Demonstration<bool P>
为POD,否则不一定如此。
A possible solution is to delegate the parameterization of POD-ness entirely to specializations of a base template base<bool P>
and have Demonstration<P>
inherit base<P>
. 一种可能的解决方案是将POD-ness的参数化完全委托给基本模板
base<bool P>
并让Demonstration<P>
继承base<P>
。 Here is an illustration: 这是一个例子:
#include<type_traits>
template<bool Param = true>
struct base // is POD
{
base() = default;
explicit constexpr base(unsigned char ch)
: _val(ch){}
unsigned char _val;
};
template<>
struct base<false> // is not POD
{
base() = default;
explicit constexpr base(unsigned char ch)
: _val(ch > 100 ? 0 : ch){}
unsigned char _val = 0;
};
template <bool MyParameter>
class Demonstration : private base<MyParameter>
{
public:
Demonstration() = default;
//trivial copy, move constructors/assignment, and trivial destructor
constexpr Demonstration(Demonstration const &) = default;
constexpr Demonstration(Demonstration &&) = default;
Demonstration & operator= (Demonstration const &) = default;
Demonstration & operator= (Demonstration &&) = default;
~Demonstration() = default;
explicit constexpr Demonstration(unsigned char toConvert)
: base<MyParameter>(toConvert)
{
}
char myValue() const {
return base<MyParameter>::_val;
}
};
#include <iostream>
using namespace std;
int main()
{
cout << "is_pod<base<true>>::value = "
<< is_pod<Demonstration<true>>::value << endl;
cout << "is_pod<base<false>>::value = "
<< is_pod<Demonstration<false>>::value << endl;
cout << "is_pod<Demonstration<true>>::value = "
<< is_pod<Demonstration<true>>::value << endl;
cout << "is_pod<Demonstration<false>>::value = "
<< is_pod<Demonstration<false>>::value << endl;
Demonstration<true> d_true(1);
Demonstration<false> d_false(101);
std::cout << "(int)Demonstration<true>(1).myValue() = "
<< (int)d_true.myValue() << endl;
std::cout << "(int)Demonstration<false>(101).myValue() = "
<< (int)d_false.myValue() << endl;
return 0;
}
Now Demonstration<P>
is POD just in case base<P>
is POD. 现在
Demonstration<P>
是POD,以防万一base<P>
是POD。 The program outputs: 该方案产出:
is_pod<base<true>>::value = 1
is_pod<base<false>>::value = 0
is_pod<Demonstration<true>>::value = 1
is_pod<Demonstration<false>>::value = 0
(int)Demonstration<true>(1).myValue() = 1
(int)Demonstration<false>(101).myValue() = 0
Built with GCC 4.8.2 and clang 3.3 使用GCC 4.8.2和clang 3.3构建
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