在运行时可以选择C ++通用类型参数吗? [英] Is it possible to choose a C++ generic type parameter at runtime?
问题描述
我想做什么是一些事情,这是一个在运行时选择类的通用类型的方法,像这样(伪代码):
Generictype type;
if(somveval == 1)
type = Integer;
if(someval == 2)
type = String;
list< type>我的列表;
这是否可能在C ++中?如果是,怎么样?
这是一个编译时间。模板参数类型必须在编译时被编译器知道。
这就是说,使用某些模板元编程技术,您可以选择一种类型或另一种AT编译时,但只有在编译时才知道所有可能的类型,并且只有在编译时可以解析选择类型的条件。
例如,使用部分专业化,您可以根据整数在编译时选择一个类型:
template< typename T>
class Foo
{};
template< int N>
struct select_type;
template<>
struct select_type< 1>
{
typedef int type;
};
template<>
struct select_type< 2>
{
typedef float type;
};
int main()
{
Foo< select_type< 1> :: type> F1; //会给你Foo< int>
Foo< select_type< 2> :: type> F2; //会给你Foo< float>
}
Is there a way to choose the generic type of a class at runtime or is this a compile-time thing in C++?
What I want to do is something like this (pseudocode):
Generictype type;
if(somveval==1)
type = Integer;
if(someval==2)
type = String;
list<type> myList;
Is this possible in C++? and if yes, how?
It's a compile time thing. Template parameter types must be known to the compiler at compile-time.
That being, said, using certain template meta-programming techniques, you can choose one type or another AT compile-time, but only if all possible types are known at compile-time, and only if the condition for selecting a type can be resolved at compile time.
For example, using partial specialization you could select a type at compile time based on an integer:
template <typename T>
class Foo
{ };
template <int N>
struct select_type;
template<>
struct select_type<1>
{
typedef int type;
};
template<>
struct select_type<2>
{
typedef float type;
};
int main()
{
Foo<select_type<1>::type> f1; // will give you Foo<int>
Foo<select_type<2>::type> f2; // will give you Foo<float>
}
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