当参数未知时如何传递函数指针

Question

我有一个静态函数Bar::Function(std::string s1, std::string s2, std::string s3 )，我想将它作为函数指针传递给具有boost::function 成员的类Foo 的构造函数。

但这三个字符串是在Foo() 类中创建的，尚不知道该函数何时作为函数指针传递给 Foo 的构造函数。 我应该像在main() 中那样简单地传递三个空字符串吗？

class Foo(boost::function<int(std::string, std::string, std::string)> fFunction)
{
public:
    Foo()
    {
        fnCallback = fFunction;
    }

    int Call()
    {
        return fnCallback("first", "second", "third")
    }
protected:
     boost::function<int(std::string, std::string, std::string)> fnCallback;
};


int main(int /*nArgc*/, char */*paszArgv*/[])
{
    boost::function<int(std::string, std::string, std::string)> fn = boost::bind(Bar::Function, "", "", "");
    Foo oFoo(fn);
    oFoo.Call();
    return 0;
}

class Bar
{
public:
    static int Function(std::string s1, std::string s2, std::string s3)
    {
        std::cout << s1 << s2 << s3 << std::endl;
    }
};

编辑：这就是代码的外观。函数指针只保存函数的地址。调用时传递参数！

fn = &Bar::Function;

Answer 1

这是一个最小的例子：

#include <iostream>
#include <string>

struct Foo
{
    typedef int (*callback_type)(std::string, std::string, std::string);

    Foo(callback_type callback)
        : _callback(callback)
    {}

    int operator()()
    {
        return _callback("1", "2", "3");
    }

private:
     callback_type _callback;
};

struct Bar
{
    static int Function(std::string s1, std::string s2, std::string s3)
    {
        std::cout << s1 << s2 << s3 << std::endl;
        return 0;
    }
};

int main()
{
    Foo foo(Bar::Function);
    foo();
    return 0;
}

打印123\n。

Answer 2

我认为您需要使用std::placeholders::_1、std::placeholders::_2、std::placeholders::_3，而不是空白字符串。如果是我，我愿意关注，

https://gist.github.com/yasuharu519/213307b4709662e60df2

#include <functional>
#include <iostream>
#include <string>

class Foo
{
public:
  Foo(std::function<int(std::string, std::string, std::string)> fFunction) : fnCallback{fFunction} {}
  int Call() { return fnCallback("first", "second", "third"); }

protected:
  std::function<int(std::string, std::string, std::string)> fnCallback;
};

class Bar
{
public:
  static int Function(std::string s1, std::string s2, std::string s3)
  {
    std::cout << s1 << s2 << s3 << std::endl;
    return 0;
  }
};

int main(int /*nArgc*/, char* /*paszArgv*/ [])
{
  using namespace std::placeholders;
  auto fn = std::bind(Bar::Function, _1, _2, _3);
  Foo oFoo(fn);
  oFoo.Call();
  return 0;
}