C++ 返回函数指针的函数
0 前言
就像C++其他类型一样，函数也拥有指针，不过不得不说C++和C的函数指针非常抽象，语法空前绝后。加之C++有C的一面，有面向对象的一面，还有面向模板的一面，在《Effective C++》里，作者第一条就点明题意，不能把C++当成1种语言来看，而是4种，每种语言都有独特的风情，而混合起来，你甚至得学习一点密码学...
接下来这段代码(来自小彭老师)，核心功能是注册GLFW的回调函数，即接受用户的键盘输入,变换相机位姿进行模型显示。
但看起来却让人望而却步。下面将对此代码进行解读。
template <class, class ...Ts>
static void (*_impl_glfw_input_callback(void (InputCtl::*pFn)(Ts...)))(GLFWwindow *, Ts...) {
static void (InputCtl::*gpFn)(Ts...);
gpFn = pFn;
return [] (GLFWwindow *window, Ts ...args) -> void {
auto game = (Game *)glfwGetWindowUserPointer(window);
if (game) [[likely]] {
(game->m_inputCtl.*gpFn)(args...);
}
};
}
template <class FpFn>
static auto glfw_input_callback(FpFn fpFn) {
return _impl_glfw_input_callback<FpFn>(fpFn());
}
// usage
glfwSetCursorPosCallback(window, glfw_input_callback([] { return &InputCtl::cursor_pos_callback; }));
1 Function Pointer in C/C++ type
1.1 ordinary function Pointer
以下这段代码来自 Author Vysandeep3
// C++ program for the above approach
#include <iostream>
using namespace std;
void demo(int& a)
{
a += 10;
}
// Driver Code
int main()
{
int num = 20;
// Now ptr contains address of demo
// function or void
void (*ptr)(int*) = &demo;
// or (*ptr)(num);
ptr(num);
cout << num << endl;
return 0;
}
returnType (*function_pointer_name)(Type a, Type b, Type ... n)
其中 function_pointer_name 定义了一个变量，他可以存储类似 returnType XXXX(Type a, Type b, Type ... n) 这种形式函数的指针。
但是有些时候我们有多个这种类型的函数，例如
int add(int a, int b);
int sub(int a, int b);
int mul(int a, int b);
int rat(int a, int b);
int (*ptr)(int, int) = NULL;
if(a == b) {
ptr = &add;
}else{
ptr = &mul;
}
我们需要在main()函数里决定什么时间什么条件一个这种类型的指针指向的函数，需要一段代码来完成这种操作。
问题是，我们可不可以写一个函数来完成这种操作呢？这也是一种重构的思想，当一段代码可能需要用到多次的时候，为什么不把他写成一个函数呢？
1.2 non-static member function of class
Its type is int (Fred::*)(char,float) if a non-static member function of class Fred
Note: if it’s a static member function of class Fred, its type is the same as if it were an ordinary function: “int (*)(char,float)”.
https://isocpp.org/wiki/faq/pointers-to-members
float (SomeClass::*my_memfunc_ptr)(int, char *);
// For const member functions, it's declared like this:
float (SomeClass::*my_const_memfunc_ptr)(int, char *) const;
my_memfunc_ptr = &SomeClass::some_member_func;
// This is the syntax for operators:
my_memfunc_ptr = &SomeClass::operator !;
// There is no way to take the address of a constructor or destructor
给出一篇学习资料： Member Function Pointers and the Fastest Possible C++ Delegates by Don Clugston
1.3 Lambda To Function Pointer
#include <iostream>
using namespace std;
#define PI(x) x, #x, x##x
auto noCapture =
[](int res) -> float
{
std::cout << "No capture lambda called with " << res << "\n";
return 99.9f;
};
typedef float(*NormalFuncType)(int);
int main(){
NormalFuncType noCaptureLambdaPtr = noCapture; //----------- (1)
float res = noCaptureLambdaPtr(100); //----------- (2)
return 0;
}
// COUT
// No capture lambda called with 100
注意这东西的地址需要用 auto noCapture = [](int res) -> float{} 来接。除此之外，就当成一个普通的函数指针就行。
给出一篇学习资料： How To Bind Lambda To Function Pointer
1.4 总结什么是指针
int* pInt;
char* pChar;
一个指针，指向一块内存中的地址(存储地址)。但是同时他又有对应的类型，char* 意为从这个地址开始读取1个字节，int* 意为从这个地址开始读取4个字节。这就是指针的核心。指针类型决定了程序如何对待一个地址。
另外C语言可以通过2个指针实现面向对象编程。当然正常的面向对象编程也是需要2个指针(*this, *underThis)。想要深入了解的话，可以搜索 opaque-pointers 这方面的知识。
给出一篇学习资料： Practical Design Patterns: Opaque Pointers and Objects in C
2 Returning a function pointer from a function in C/C++
以下这段代码来自 Author Vysandeep3
#include <iostream>
using namespace std;
int add(int a, int b) {
return a + b;
}
int subtract(int a, int b) {
return a - b;
}
int (*get_operation(char op))(int, int) {
if (op == '+') {
return &add;
} else if (op == '-') {
return &subtract;
} else {
return NULL;
}
}
int main() {
int (*op)(int, int) = get_operation('+');
int result = op(3, 4);
cout << "Result: " << result << endl;
return 0;
}
int (*get_operation(char op))(int, int):
- 其中 get_operation(char op) 是一个返回函数指针的函数
- int (*) (int, int) 是返回的函数指针所指向的函数类型
这东西看起来确实很怪..., 但是我们只能接受。
这里给出一种理解方式, 首先一个指针需要两个标识符 Type* ptr_name：
int* ptr; // ptr is a pointer to an integer
int(*)(int, int); // key idea: function pointer type
// ptr lost a pointerType like int*
int (*ptr)(int, int); // ptr is a pointer to a function that takes that takes two arguments and returns an integer
// int(*)(int, int) ptr;
//---------------------------------------------------------------------//
int ptr(char op); // ptr is a function that takes that takes one char type argument and returns an integer
// ptr() lost a returnType like int
int (*ptr(char op))(int, int){}; // ptr() is a function that takes one char argument returns a pointer to a function which two arguments and returns an integer.
// int(*)(int, int) ptr(char op) {};
https://www.learncpp.com/cpp-tutorial/introduction-to-pointers/
3. C - Variable Arguments (Variable length arguments)
printf("Some values: %d, %s, %c!", 4, "foo", 'z')
#include <stdarg.h>
void my_printf(char* format, ...)
{
va_list argp;
va_start(argp, format);
while (*format != '\0') {
if (*format == '%') {
format++;
if (*format == '%') {
putchar('%');
} else if (*format == 'c') {
char char_to_print = va_arg(argp, int);
putchar(char_to_print);
} else {
fputs("Not implemented", stdout);
}
} else {
putchar(*format);
}
format++;
}
va_end(argp);
}
The C library macro void va_start(va_list ap, last_arg) initializes ap variable to be used with the va_arg and va_end macros. The last_arg is the last known fixed argument being passed to the function i.e. the argument before the ellipsis.
https://www.tutorialspoint.com/cprogramming/c_variable_arguments.htm
https://jameshfisher.com/2016/11/23/c-varargs/
https://www.tutorialspoint.com/c_standard_library/c_macro_va_start.htm
4. Variadic Template
C++ Primer P700.
这个东西说白了，就是类似C - Variable Arguments，可以接收任意长度的函数参数，不过与C - Variable Arguments这种需char* format来自己告知函数对应参数的类型。Variadic Template 会自动生成相应的函数定义以及声明，这是模板编程的优势。详情看下面的实例代码。
// Args is a template parameter pack; rest is a function parameter pack
// Args represents zero or more template type parameters
// rest represents zero or more function parameters
template <typename T, typename... Args>
void foo(const T &t, const Args& ... rest);
int i = 0; double d = 3.14; string s = "how now brown cow";
foo(i, s, 42, d); // three parameters in the pack
foo(s, 42, "hi"); // two parameters in the pack
foo(d, s); // one parameter in the pack
foo("hi"); // empty pack
the compiler will instantiate four different instances of foo:
void foo(const int&, const string&, const int&, const double&);
void foo(const string&, const int&, const char(&)[3]);
void foo(const double&, const string&);
void foo(const char(&)[3]);
In each case, the type of T is deduced from the type of the first argument. The
remaining arguments (if any) provide the number of, and types for, the additional
arguments to the function.
#include<iostream>
using namespace std;
template<typename ... Args> void g(Args ... args) {
cout << sizeof...(Args) << endl; // number of type parameters
cout << sizeof...(args) << endl; // number of function parameters
}
int main(){
g(1,2,3,4);
return 0;
}
/*
* 4
* 4
*/
5 Variadic Template with member function pointer
当 Variadic Template 来接收 member function pointer时，不需要显式的声明成员函数的参数类型，编译器会自动推导。
#include <cstdio>
class A{
public:
void func(int xpos, int ypos);
};
void A::func(int xpos, int ypos){
printf("Hello World!");
}
template <class ...Ts>
void (* Test(void (A::*pFn)(Ts...)))(Ts ...){
return nullptr;
};
/* First instantiated from: insights.cpp:19 */
#ifdef INSIGHTS_USE_TEMPLATE
template<>
void (*Test<int, int>(void (A::*pFn)(int, int)))(int, int)
{
return nullptr;
}
#endif
;
int main()
{
A a;
Test(&A::func); // line == 19
return 0;
}
https://cppinsights.io/
https://adroit-things.com/programming/c-cpp/how-to-bind-lambda-to-function-pointer/
https://www.codeproject.com/Articles/7150/Member-Function-Pointers-and-the-Fastest-Possible
6 最终解析
template <class, class ...Ts>
static void (*_impl_glfw_input_callback(void (InputCtl::*pFn)(Ts...)))(GLFWwindow *, Ts...) {
static void (InputCtl::*gpFn)(Ts...);
gpFn = pFn;
return [] (GLFWwindow *window, Ts ...args) -> void {
auto game = (Game *)glfwGetWindowUserPointer(window);
if (game) [[likely]] {
(game->m_inputCtl.*gpFn)(args...);
}
};
}
template <class FpFn>
static auto glfw_input_callback(FpFn fpFn) {
return _impl_glfw_input_callback<FpFn>(fpFn());
}
// usage
glfwSetCursorPosCallback(window, glfw_input_callback([] { return &InputCtl::cursor_pos_callback; }));
-
glfw_input_callback([] { return &InputCtl::cursor_pos_callback; })
传入一个lambda函数指针, 类型使用template <class FpFn>的FpFn自动定义，函数指针值使用fpFn承接。 -
_impl_glfw_input_callback<FpFn>(fpFn());
fpFn()调用匿名函数，返回&InputCtl::cursor_pos_callback成员函数指针。 -
Variadic Template with member function pointer
template <class, class ...Ts>
static void (*_impl_glfw_input_callback(void (InputCtl::*pFn)(Ts...)))(GLFWwindow *, Ts...)
_impl_glfw_input_callback(void (InputCtl::*pFn)(Ts...)) 使用模板自动承接相应的成员函数指针，不必明确指出函数的参数等信息。
- 函数调用
return [] (GLFWwindow *window, Ts ...args) -> void {
// Game class 的 *this 指针
auto game = (Game *)glfwGetWindowUserPointer(window);
if (game) [[likely]] {
// 成员函数调用
(game->m_inputCtl.*gpFn)(args...);
}
};
注册回调函数的核心无非就是执行回调函数中的代码。
X.Refference
- Author Vysandeep3
- https://isocpp.org/wiki/faq/pointers-to-members
- Member Function Pointers and the Fastest Possible C++ Delegates by Don Clugston
- How To Bind Lambda To Function Pointer
- Practical Design Patterns: Opaque Pointers and Objects in C
- Author Vysandeep3
- https://www.learncpp.com/cpp-tutorial/introduction-to-pointers/
- https://www.tutorialspoint.com/cprogramming/c_variable_arguments.htm
- https://jameshfisher.com/2016/11/23/c-varargs/
- https://www.tutorialspoint.com/c_standard_library/c_macro_va_start.htm
- https://cppinsights.io/
- https://adroit-things.com/programming/c-cpp/how-to-bind-lambda-to-function-pointer/
- https://www.codeproject.com/Articles/7150/Member-Function-Pointers-and-the-Fastest-Possible
- 小彭老师 OPENGL 课程实验源代码
如果我的工作对您有帮助，您想回馈一些东西，你可以考虑通过分享这篇文章来支持我。我非常感谢您的支持，真的。谢谢！
作者：Dba_sys (Jarmony)
转载以及引用请注明原文链接：https://www.cnblogs.com/asmurmur/p/17826429.html
本博客所有文章除特别声明外，均采用CC 署名-非商业使用-相同方式共享 许可协议。
