首先，您的示例不应编译。

int someFunction(std::function<int(int)> sum) {
  return sum(1,3);
}

/*
std::function<int(int)>
              ^    ^
              |    |
           return  parameter
            type  

So, calling sum(1,3) would require function<int(int, int) as you are passing two parameters and returning a value.
*/

更多示例

/*
The structure is "return_type(param1, param2, ..)" for the function and the following 
`[]` is used for variable capture (by value [x] or by reference [&x]) that resides out the function.
*/

// takes one param and returns nothing
function<void(int)> print = [](auto i) {
   court<<i<<endl;
}

// takes two params and returns the sum
function<int(int, int)> sum = [](auto i, auto j) {
   return i + j;
}

// takes one param and returns with added 2
function<int(int)> sum = [](auto i) {
   return i + 2;
}

我们如何定义采用“int（int，int）"这样的模板类型的类？

template<typename T>
class Class {
  public:
    void call(T sum) {
      cout << sum(1, 3) << endl;
    } 
};

// usage
auto sum = [](auto a, auto b) { return a + b; };
auto instance = new Class<decltype(sum)>();
instance->call(sum);

感谢@Raildex的评论，我回答了我自己的问题。基本上，问题中的int（int，int）类型将引用一个函数指针，该指针接受2个int并返回一个int。
因此，示例类可以是：

template<typename T>
class SomeClass {
  public:
    void someMethod(T sum) {
      cout << sum(1, 3) << endl;
    } 
};

SomeClass someClass;
// the template type is not required here, but has been added just for clarity.
someClass.someMethod<int(int, int)>([](int a, int b){return a + b;});