假设我们正在讨论的所有operator()和函数都不是模板或重载：

template <typename T>
struct get_arity : get_arity<decltype(&T::operator())> {};
template <typename R, typename... Args>
struct get_arity<R(*)(Args...)> : std::integral_constant<unsigned, sizeof...(Args)> {};
// Possibly add specialization for variadic functions
// Member functions:
template <typename R, typename C, typename... Args>
struct get_arity<R(C::*)(Args...)> :
    std::integral_constant<unsigned, sizeof...(Args)> {};
template <typename R, typename C, typename... Args>
struct get_arity<R(C::*)(Args...) const> :
    std::integral_constant<unsigned, sizeof...(Args)> {};

// Add all combinations of variadic/non-variadic, cv-qualifiers and ref-qualifiers

Demo。

几年后，但看到我的完整（免费）解决方案here（生产级，完全文档化）。在您的情况下，您需要帮助模板“ArgCount_v”（在上面的链接中有完整的文档）。将其应用于您自己的代码（显示预期的结果-请参阅here）：

#include <iostream>

// See https://github.com/HexadigmSystems/FunctionTraits
#include "TypeTraits.h" 

template<typename TFunc>
void printArity(TFunc mFunc)
{
    ////////////////////////////////////////////////
    // Everything in "TypeTraits.h" above declared
    // in this namespace
    ////////////////////////////////////////////////
    using namespace StdExt;

    /////////////////////////////////////////////////////  
    // Note: "tcout" (declared in "TypeTraits.h" above)
    // always resolves to "std::cout" on non-Microsoft
    // platforms and usually "std::wcout" on Microsoft
    // platforms (when compiling for UTF-16 in that
    // environment which is usually the case). You can
    // directly call "std::cout" or "std::wcout" instead
    // if you prefer, assuming you know which platform
    // you're always running on.
    /////////////////////////////////////////////////////  
    tcout << "arity: " << ArgCount_v<TFunc> << std::endl;
}

void testFunc(int) { }

int main()
{
    printArity([](){}); // prints 0
    printArity([&](int x, float y){}); // prints 2
    printArity(testFunc); // prints 1

    return 0;
}