在 C++0x 中专门针对 lambda 的模板 [英] Specializing a template on a lambda in C++0x

问题描述

我编写了一个 traits 类，它让我可以在 C++0x(用 gcc 4.5.0 测试)中提取有关函数或函数对象的参数和类型的信息.一般情况处理函数对象:

I've written a traits class that lets me extract information about the arguments and type of a function or function object in C++0x (tested with gcc 4.5.0). The general case handles function objects:

template <typename F>
struct function_traits {
    template <typename R, typename... A>
    struct _internal { };

    template <typename R, typename... A>
    struct _internal<R (F::*)(A...)> {
        // ...
    };

    typedef typename _internal<decltype(&F::operator())>::<<nested types go here>>;
};

然后我专门研究了全局范围内的普通函数:

Then I have a specialization for plain functions at global scope:

template <typename R, typename... A>
struct function_traits<R (*)(A...)> {
    // ...
};

这很好用，我可以将函数传递到模板或函数对象中，并且它可以正常工作:

This works fine, I can pass a function into the template or a function object and it works properly:

template <typename F>
void foo(F f) {
    typename function_traits<F>::whatever ...;
}

int f(int x) { ... }
foo(f);

如果不是将函数或函数对象传递给 foo，我想传递一个 lambda 表达式怎么办?

What if, instead of passing a function or function object into foo, I want to pass a lambda expression?

foo([](int x) { ... });

这里的问题是 function_traits<> 的特化都不适用.C++0x 草案说表达式的类型是唯一的、未命名的、非联合类类型".对在表达式上调用 typeid(...).name() 的结果进行分解给了我似乎是 gcc 对 lambda 的内部命名约定，main::{lambda(int)#1}，不是在语法上代表 C++ 类型名的东西.

The problem here is that neither specialization of function_traits<> applies. The C++0x draft says that the type of the expression is a "unique, unnamed, non-union class type". Demangling the result of calling typeid(...).name() on the expression gives me what appears to be gcc's internal naming convention for the lambda, main::{lambda(int)#1}, not something that syntactically represents a C++ typename.

简而言之，我可以在此处放入模板吗:

In short, is there anything I can put into the template here:

template <typename R, typename... A>
struct function_traits<????> { ... }

这将允许这个 traits 类接受一个 lambda 表达式吗?

that will allow this traits class to accept a lambda expression?

推荐答案

我认为可以专门化 lambda 的特征并对未命名函子的签名进行模式匹配.这是适用于 g++ 4.5 的代码.尽管它有效，但 lambda 上的模式匹配似乎与直觉相反.我有内联评论.

I think it is possible to specialize traits for lambdas and do pattern matching on the signature of the unnamed functor. Here is the code that works on g++ 4.5. Although it works, the pattern matching on lambda appears to be working contrary to the intuition. I've comments inline.

struct X
{
  float operator () (float i) { return i*2; }
  // If the following is enabled, program fails to compile
  // mostly because of ambiguity reasons.
  //double operator () (float i, double d) { return d*f; } 
};

template <typename T>
struct function_traits // matches when T=X or T=lambda
// As expected, lambda creates a "unique, unnamed, non-union class type" 
// so it matches here
{
  // Here is what you are looking for. The type of the member operator()
  // of the lambda is taken and mapped again on function_traits.
  typedef typename function_traits<decltype(&T::operator())>::return_type return_type;
};

// matches for X::operator() but not of lambda::operator()
template <typename R, typename C, typename... A>
struct function_traits<R (C::*)(A...)> 
{
  typedef R return_type;
};

// I initially thought the above defined member function specialization of 
// the trait will match lambdas::operator() because a lambda is a functor.
// It does not, however. Instead, it matches the one below.
// I wonder why? implementation defined?
template <typename R, typename... A>
struct function_traits<R (*)(A...)> // matches for lambda::operator() 
{
  typedef R return_type;
};

template <typename F>
typename function_traits<F>::return_type
foo(F f)
{
  return f(10);
}

template <typename F>
typename function_traits<F>::return_type
bar(F f)
{
  return f(5.0f, 100, 0.34);
}

int f(int x) { return x + x;  }

int main(void)
{
  foo(f);
  foo(X());
  bar([](float f, int l, double d){ return f+l+d; });
}

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