[英]C++ How to Reference Templated Functions using std::bind / std::function
If you have a templated class or a templated function, (or combination of the two), how do you bind that function, (preserving the template type parameter)? 如果你有一个模板化的类或模板化的函数(或两者的组合),你如何绑定该函数,(保留模板类型参数)?
I was given some help about the basic syntax in a post below, to bind to functions with explicit template type parameters, but lose the ability to provide template type parameters in the process. 我在下面的帖子中给出了一些关于基本语法的帮助,用显式模板类型参数绑定到函数,但是在这个过程中失去了提供模板类型参数的能力。
Is it possible to get this to work so that it is still possible to provide template type parameters with future calls? 是否有可能使其工作,以便仍然可以提供未来调用的模板类型参数?
Cleaned up this code a lot, but it obviously won't compile because I can't find the correct syntax, (are there any ways of doing this)? 清理过这段代码很多,但显然无法编译,因为我找不到正确的语法,(有没有办法做到这一点)?
Removed the "vector" requirement to simplify this: 删除了“向量”要求以简化此操作:
Thanks for the help! 谢谢您的帮助!
#include <functional>
#include <vector>
#include <string>
/***************************************/
template <typename CommandTemplateType>
class Storage
{
public:
// No idea how to define this vector to allow Template Parameters
// static std::vector<std::function<void<ParameterTemplateType>
// (std::shared_ptr<ParameterTemplateType>)>> Functions;
// I really don't need the collection, a single member would kick start my research:
static std::function<void<ParameterTemplateType>(std::shared_ptr<ParameterTemplateType>)> Function;
template <typename ParameterTemplateType>
static void Execute(ParameterTemplateType parameter)
{
// Look up index, or loop through all..
// I am trying to invoke the bound function with a template param:
// Functions[index]<ParameterTemplateType>(parameter);
// preferably, just:
Function<ParameterTempalteType>(parameter);
}
};
/***************************************/
template <typename TemplateType>
class MyClass
{
template <typename ParameterTemplateType>
void MyFunction(ParameterTemplateType myParameter)
{
// Do something;
}
MyClass()
{
std::string parameter = L"Test String";
// Do not know how to include the
// template<typename ParameterTemplateType> definition to bind call.
// Storage::Functions.push_back(
// std::bind(&MyClass::MyFunction<ParameterTemplateType>,
// this, std::placeholders::_1));
// Or just something like:
Storage::Function = std::bind(&MyClass::MyFunction<ParameterTemplateType>,
this, std::placeholders::_1));
/***************************************/
// Call the bound function with an explicit parameter somehow:
std::string parameter = L"Test String";
Storage::Execute<std::string>(parameter);
}
};
The key issues is that in C++11 you cannot do something like: 关键问题是在C ++ 11中你不能做类似的事情:
// Doesn't compile
template <typename TemplateType>
static std::function<void(std::shared_ptr<TemplateType>)> Function;
Classes and Functions can be templated, but not member properties. 类和函数可以是模板化的,但不是成员属性。
The "Magic" is: “魔术”是:
/*******************************************************************/
// Define a Function Pointer in a Container
class Storage
{
template <typename TemplateType>
struct FunctionContainer {
static std::function<void(std::shared_ptr<TemplateType>)> Function;
};
};
/*******************************************************************/
// Initialize FunctionContainer's Static Function Pointer if using static pointer.
template <typename TemplateType>
std::function<void(std::shared_ptr<TemplateType>)> Storage
::FunctionContainer<TemplateType>::Function;
You can then Bind a templated function to this function like: 然后,您可以将模板化函数绑定到此函数,如:
// Bind Function Pointer in Container to a Local Function
class MyClass
{
template <typename TemplateType>
void MyFunction(std::shared_ptr<TemplateType> parameter)
{
// Do something.
// You can make this templated or non-templated.
}
MyClass()
{
// If you really want, you can templatize std::string in the following:
Storage::FunctionContainer<std::string>::Function
= std::bind(&MyFunction<std::string>, this, std::placeholders::_1);
}
}
And you can invoke all of this and provide a templated type parameter like so: 您可以调用所有这些并提供模板化类型参数,如下所示:
//Invocation
std::shared_ptr<std::string> parameter;
parameter->get() = "Hello World".
Storage::FunctionContainer<std::string>::Function(parameter);
The template argument for std::function
should be the signature of the function after template type substitution has been done. 在完成模板类型替换之后 ,
std::function
的模板参数应该是std::function
的签名。 In your case, neither TemplateType
nor FunctionTemplateType
have an effect on the signature of the member function MyFunction
- it will always return a std::string
and take a single std::string
argument. 在您的情况下,
TemplateType
和FunctionTemplateType
都不会影响成员函数MyFunction
的签名 - 它将始终返回std::string
并获取单个std::string
参数。 Therefore, the std::function
you're going to store in your std::vector
should be: 因此,您要存储在
std::vector
的std::function
应该是:
static std::vector<std::function<std::string(std::string)>> Functions;
Recall that a member function has an implicit first argument this
. 回想一下,一个成员函数具有一个隐含的第一个参数
this
。 You need to bind the first argument of MyClass<...>::MyFunc<...>
to the object you want it to be called on. 您需要将
MyClass<...>::MyFunc<...>
的第一个参数绑定到要调用它的对象。 Presumably, since you're binding the function in MyClass
's constructor, you want the object to be that MyClass
instance. 据推测,既然您在
MyClass
的构造函数中绑定了该函数,那么您希望该对象成为MyClass
实例。 That means your push_back
should look like this: 这意味着你的
push_back
应如下所示:
Storage::Functions.push_back(
std::bind(&MyClass<TemplateType>::MyFunction<int>, this,
std::placeholders::_1)
);
Now the function that is pushed into Functions
is bound to your MyClass
object and takes a single argument of type std::string
. 现在,推入
Functions
绑定到MyClass
对象,并采用类型为std::string
的单个参数。 You can call one of these functions like so: 您可以像这样调用以下函数之一:
Storage::Functions[0]("something");
If I understood you right... :) 如果我理解你的话...... :)
What you want to do is not possible because for template<class T> void foo(T) functions foo<int>() and foo<double> are of different types and you can not create vector holding pointers to both that functions directly because vector is homogeneous container. 您想要做的是不可能的,因为对于模板<class T> void foo(T)函数foo <int>()和foo <double>属于不同的类型,并且您不能创建向量保持指向这两个函数的向量,因为矢量是同质的容器。
To overcome that we can use boost::variant<> to either store pointers to different types of functions or to store arguments of functions. 为了克服这个问题,我们可以使用boost :: variant <>来存储指向不同类型函数的指针或存储函数的参数。
template<class T> void foo(T);
typedef boost::variant<void (*)(int), void (*)(double)> func_ptr_variant;
std::vector<func_ptr_variant> v;
v.push_back(foo<int>);
v.push_back(foo<double>);
typedef boost::variant<int, double> argument;
std::vector<void (*)(argument)) v;
v.push_back(foo);
v.push_back(bar);
// foo and bar are defined as void foo(argument a) and void bar(argument a)
Unfortunately, in any case you will need to instantiate function templates before inserting them to container because C++ can not do code generation on the fly. 不幸的是,在任何情况下,您都需要在将函数模板插入容器之前对其进行实例化,因为C ++无法动态执行代码生成。 I think that it is possible that you know all possible types of arguments that the function may be used with so that may be not a problem.
我认为您可能知道函数可能使用的所有可能类型的参数,因此可能不是问题。
MyClass's c-tor doesn't know anything about FunctionTemplateType
that's why it can push_back only explicit specialized (sorry, it's term of mine... I don't know the right term) like this MyClass的c-tor对
FunctionTemplateType
一无所知,这就是为什么它只能push_back 显式专门 (对不起,这是我的术语......我不知道正确的术语),就像这样
#include <functional>
#include <vector>
#include <string>
struct Storage
{
// Have no idea what this signature should really be:
static std::vector<std::function<void ()>> Functions;
};
std::vector<std::function<void ()>> Storage::Functions;
template <typename TemplateType>
class MyClass
{
template <typename FunctionTemplateType>
std::string MyFunction(std::string myParameter)
{
return "Hellö: " + myParameter;
}
public:
MyClass()
{
Storage::Functions.push_back(
std::bind( & MyClass<TemplateType>::MyFunction<std::string>, this, "borisbn" )
// ^^^^^^^^^^^
);
}
};
int main() {
MyClass<int> obj;
}
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