[英]C++: Function pointer to functions with variable number of arguments
I'm trying to figure out a way of how to be able to assign a function pointer to functions with different number of arguments. 我试图找出一种方法,如何能够为具有不同数量的参数的函数分配函数指针。
I have a while loop which takes a number of different functions as a conditional statement, so instead of writing multiple while loops with exactly the same code inside I'd like to have one with a function pointer. 我有一个while循环,它将许多不同的函数作为一个条件语句,所以我没有用完全相同的代码编写多个while循环,而是希望有一个带有函数指针的函数。 All the functions are of format bool f(...)
. 所有函数都是bool f(...)
格式。 I think some code will best illustrate what I mean: 我认为一些代码最能说明我的意思:
int a, b, c, d;
MyClass* my_class;
typedef bool (MyClass::*my_fun_t)();
my_fun_t my_fun;
if (condition1)
my_fun = &MyClass::function_one();
else if (condition2)
my_fun = &MyClass::function_two(a, b);
else if (condition3)
my_fun = &MyClass::function_three(a, b, c);
else if (condition4)
my_fun = &MyClass::function_four(a, b, c, d);
while ((my_class->*my_fun)())
{ ... }
Now this obviously doesn't work because the functions have different signatures. 现在这显然不起作用,因为函数具有不同的签名。 Is it at all possible to make it work in a similar fashion? 是否可以以类似的方式使其工作? Are functoids something I should look at? 我应该看一下functoids吗?
You could use std::function<>
and std::bind()
. 您可以使用std::function<>
和std::bind()
。
#include <functional>
using std::placeholders::_1;
typedef std::function<bool(MyClass&)> my_fun_t;
my_fun_t my_fun;
if (condition1)
my_fun = std::bind(&MyClass::function_one, _1);
else if (condition2)
my_fun = std::bind(&MyClass::function_two, _1, a, b);
else if (condition3)
my_fun = std::bind(&MyClass::function_three, _1, a, b, c);
else if (condition4)
my_fun = std::bind(&MyClass::function_four, _1, a, b, c, d);
while (my_fun(my_class)) { ... }
These assumes you will use C++11. 这些假设您将使用C ++ 11。 If you can't use C++11 but can use TR1, replace all std::
with std::tr1::
. 如果您不能使用C ++ 11但可以使用TR1,请将所有std::
替换为std::tr1::
。 There is also a Boost implementation . 还有一个Boost实现 。
This works for me: 这对我有用:
#include <iostream>
#include <cstdarg>
using namespace std;
class MyInterface
{
public:
virtual bool func(int argc, ...) = 0;
};
class MyImpl : public MyInterface
{
public:
virtual bool func(int argc, ...);
};
bool MyImpl::func(int argc, ...)
{
va_list varargs;
va_start(varargs,argc);
cout << "Arguments passed:" << endl;
for(int i = 0; i < argc; ++i)
{
// expect double values
double val = va_arg(varargs,double);
cout << val << endl;
}
va_end(varargs);
return true;
}
typedef bool (MyInterface::*MyFunc)(int, ...);
int main() {
MyImpl impl;
MyInterface* interface = &impl;
MyFunc pfunc = &MyInterface::func;
if(!(interface->*pfunc)(2,double(3.14),double(2.72)))
{
return 1;
}
return 0;
}
Output: 输出:
Arguments passed:
3.14
2.72
Obviously you CAN declare and use function pointers for (member-)functions using variable arguments. 显然,您可以使用变量参数为(member-)函数声明和使用函数指针。
You want std::function
, a polymorphic function object, and std::bind
to create function objects by binding arguments to the parameters of other functors. 您希望std::function
,一个多态函数对象和std::bind
通过将参数绑定到其他仿函数的参数来创建函数对象。
If you can't use C++11, then boost::function
and boost::bind
are equivalent, although slightly more restrictive. 如果你不能使用C ++ 11,那么boost::function
和boost::bind
是等价的,虽然稍微有些限制。
std::function<bool()> my_fun;
if (condition1)
my_fun = std::bind(&MyClass::function_one, my_class);
else if (condition2)
my_fun = std::bind(&MyClass::function_two, my_class, a, b);
else if (condition3)
my_fun = std::bind(&MyClass::function_three, my_class, a, b, c);
else if (condition4)
my_fun = std::bind(&MyClass::function_four, my_class, a, b, c, d);
while (my_fun())
{ ... }
I'm trying to figure out a way of how to be able to assign a function pointer to functions with different number of arguments. 我试图找出一种方法,如何能够为具有不同数量的参数的函数分配函数指针。
You can't. 你不能。 Function pointers are specific to one function signature. 函数指针特定于一个函数签名。 This is entirely logical: how would you invoke such a function? 这完全合乎逻辑:你将如何调用这样的函数? (Yes, C allows invoking functions without specifying in their declaration how many parameters the function has, but this doesn't work in C++ since it subverts the type system.) (是的,C允许调用函数而不在其声明中指定函数有多少参数,但这在C ++中不起作用,因为它颠覆了类型系统。)
Are functoids something I should look at? 我应该看一下functoids吗?
Generally yes, but they don't solve this problem. 一般是的,但他们没有解决这个问题。
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