问题描述
由于我的last最近的问题不幸的措辞，并导致解决另一个问题，然后我的，在这里我将尝试制定我的实际问题，以明确的方式。
在我们开始之前，作为一个旁注，我整合了JavaScript引擎V8到我的C ++应用程序。这就是在本例中所有类型从何而来。而这也是我需要在最后一个原函数指针的事实的原因。但我细说了下文。
从类中我需要通过一个Lambda前pression与捕获子句 [=] 作为类型参数 STD ::功能给另一个函数，并将其转换为原始函数指针那里。
在此code， InvocationCallback 简直就是与签名功能的typedef 处理＆LT;价值＆GT;（参数常量和放大器;） 。
的typedef处理＆LT; VALUE＆GT; （* InvocationCallback）（参数常量和放大器;）;无效绑定（字符串名称，功能和LT;处理＆LT; VALUE＆GT;（参数常量和放大器;）＆GT;功能）
{
InvocationCallback *功能= Function.target＆LT; InvocationCallback＆GT;（）;
}
所有的lambda前pressions具有相同的签名，太。需要注意的是处理＆LT;弦乐＆GT; 是兼容处理＆LT;价值＆GT; 在这个例子中。它是由JavaScript引擎V8给出了。
绑定（名，[=]（常量参数和放大器;参数） - ＆GT;处理＆LT; VALUE＆GT; {
手柄＆LT;串GT;结果=字符串::新（名称（）c_str（））;
返回结果;
}）;
C ++允许我到这个拉姆达为的std ::功能传递给函数的上方。但我想一个lambda前pression还存储参考对象是指。不知怎的，由指定的访问[=] 必须实现。这可能是铸造的原因的std ::功能来的原始函数指针失败。
InvocationCallback *功能= Function.target＆LT; InvocationCallback＆GT;（）;
有既不是一个编译时错误，也不是一个运行时错误，但调试器告诉我，这导致空指针。但我需要做进一步处理原始函数指针。我想我可以在的std ::绑定拉姆达转换ING引用或这个终场第一。
更新：由于这似乎是不可能得到国家走出拉姆达的，这就是我试过了。它编译但函数出来是一个空指针。
绑定（名，对此，[]（基*的对象，常量参数和放大器;参数） - ＆GT;处理＆LT; VALUE＆GT; {
返回V8 ::字符串::新（（（*派生）对象） - 方式＆gt;名称（）c_str（））;
}）;无效绑定（字符串名称，模块*对象，功能和LT;处理＆LT; VALUE＆GT;（模块*，参数常量和放大器;）＆GT;功能）
{
功能＆LT;＆手柄LT; VALUE＆GT;（参数常量和放大器;）＆GT;方法=的std ::绑定（函数，对象的std ::占位符:: _ 1）;
InvocationCallback *功能= method.target＆LT; InvocationCallback＆GT;（）;}
您不能，因为它可以捕获一个lambda是的闭合的，所以它有状态（它是与实例变量的对象）。一个函数指针没有状态。因此，你不能没有做到这一点无论1）您所使用的API需要的函数指针也可以让你通过，你传递状态的用户数据参数，或者2）存储在一个全局变量或东西的状态。
周围堆栈溢出搜索成员函数回调，你会得到一个想法（基本上，你是想用一个成员函数时，运算符（），作为回调）。
Since my last recent question was unfortunately worded and resulted in a solution to another problem then mine, here I will try to formulate my actual problem in a clear way.
Before we start, as a sidenote, I am integrating the Javascript Engine V8 into my C++ application. That's where all the types in the example come from. And that's also the reason for the fact that I need a raw function pointer in the end. But I elaborate on this below.
From inside a class I need to pass a lambda expression with the capture clause [=] as parameter of the type std::function to another function and cast it to a raw function pointer there.
In this code, InvocationCallback is simply a typedef for a function with the signature Handle<Value>(Arguments const &).
typedef Handle<Value> (*InvocationCallback)(Arguments const &);
void Bind(string Name, function<Handle<Value>(Arguments const &)> Function)
{
InvocationCallback* function = Function.target<InvocationCallback>();
}
All the lambda expressions have the same signature, too. Note that Handle<String> is compatible to Handle<Value> in this example. It's given by the Javascript Engine V8, too.
Bind("name", [=](const Arguments& args) -> Handle<Value>{
Handle<String> result = String::New(Name().c_str());
return result;
});
C++ allows me to pass this lambda as std::function to the function above. But I guess a lambda expression also stores a reference to object it refers. Somehow the access specified by [=] must be realized. That might be the reasons that casting the std::function to a raw function pointer fails.
InvocationCallback* function = Function.target<InvocationCallback>();
There is neither a compile time error nor a runtime error but the debugger tells me that it results in a null pointer. But I need the raw function pointer for further processing. I guess I could convert the lambda after std::binding the reference or this-pointer first.
Update: Since it seems to be impossible to get the state out of the lambda, this is what I tried. It compiles but function come out to be a null pointer.
Bind("name", this, [](Base* object, const Arguments& args) -> Handle<Value>{
return v8::String::New(((Derived*)object)->Name().c_str());
});
void Bind(string Name, Module *Object, function<Handle<Value>(Module*, Arguments const &)> Function)
{
function<Handle<Value>(Arguments const &)> method = std::bind(Function, Object, std::placeholders::_1);
InvocationCallback* function = method.target<InvocationCallback>();
}
You can't, because a lambda which captures is a closure, so it has state (it is an object with instance variables). A function pointer has no state. Thus, you cannot do this without either 1) the API you are using that requires the function pointer also allows you to pass a user data argument where you pass the state, or 2) storing the state in a global variable or something.
Search around Stack Overflow for "member function to callback" and you will get an idea (basically, you are wanting to use a member function, the operator(), as a callback).
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