I have a tricky problem and I’m working on it for several hours but can’t find a cause and solution of it. Hope someone help me.
I have to demonstrate function being called inside another function( pls see the comment in seminar.cpp)
Below are the files ( I have separated it into header and code files)
main.cpp
#include <iostream>
#include <functional>
#include "seminar.h"
int main()
{
Tom::Car::Car car;
Nor::Driving drivingnow;
std::vector<uint8_t> X = car.road(drivingnow);
for(int i = 0 ; i < X.size() ; i++){
std::cout<<unsigned(X[i])<<" ";
}
return 0;
}
seminar.h
#pragma once
#include "dist.h"
#include <vector>
#include <bits/stdc++.h>
namespace Tom
{
namespace Car
{
class Car
{
public:
std::vector<uint8_t> road(Nor::Driving &driving);
};
} // namespace Car
} // namespace Tom
seminar.cpp
#include "seminar.h"
#include <algorithm>
#include <functional>
namespace Tom
{
namespace Car
{
std::vector<uint8_t> drive(Nor::Range &range)
{
std::vector<uint8_t> s;
s.push_back(range.z);
s.push_back(range.zz);
return s;
}
template <typename T, typename B, typename L>
std::vector<uint8_t> Content(T Sec, B Byte, L Func)
{
Nor::Range Rom;
std::vector<uint8_t> z = Func(Rom);
return z;
}
std::vector<uint8_t> Car::road(Nor::Driving &driving)
{
std::function<std::vector<uint8_t>(Nor::Range &)> Func = &drive;
return Content(driving, 1, Func); // passing drive function into content
}
} // namespace Car
} // namespace Tom
dist.h
namespace Nor
{
class Driving{
public:
int x = 1;
};
class Range{
public:
int z = 50;
int zz = 100;
};
}
The above code and file structure works correctly and give me the correct expected output ie 50 100
Live here
Now I want to do more separation ie I want the implementation of drive function to move in another file ie in type.cpp
type.cpp
#include <algorithm>
#include "seminar.h"
#include <functional>
namespace Tom
{
namespace Car
{
std::vector<uint8_t> Car::drive(Nor::Range &range)
{
std::vector<uint8_t> s;
s.push_back(range.z);
return s;
}
} // namespace Car
} // namespace Tom
seminar.h
#pragma once
#include "dist.h"
#include <vector>
#include <bits/stdc++.h>
namespace Tom
{
namespace Car
{
class Car
{
public:
std::vector<uint8_t> road(Nor::Driving &driving);
std::vector<uint8_t> drive(Nor::Range &range);
};
} // namespace Car
} // namespace Tom
seminar.cpp
#include "seminar.h"
#include <algorithm>
#include <functional>
namespace Tom
{
namespace Car
{
template <typename T, typename B, typename L>
std::vector<uint8_t> Content(T Sec, B Byte, L Func)
{
Nor::Range Rom;
std::vector<uint8_t> z = Func(Rom);
return z;
}
std::vector<uint8_t> Car::road(Nor::Driving &driving)
{
std::function<std::vector<uint8_t>(Nor::Range &)> Func = &drive;
return Content(driving, 1, Func);
}
} // namespace Car
} // namespace Tom
Live here
After doing this I am getting an below error:
seminar.cpp: In member function ‘std::vector<unsigned char> Tom::Car::Car::road(Nor::Driving&)’:
seminar.cpp:22:71: error: ISO C++ forbids taking the address of an unqualified or parenthesized non-static member function to form a pointer to member function. Say ‘&Tom::Car::Car::drive’ [-fpermissive]
22 | std::function<std::vector<uint8_t>(Nor::Range &)> Func = &drive;
| ^~~~~
seminar.cpp:22:71: error: conversion from ‘std::vector (Tom::Car::Car::*)(Nor::Range&)’ to non-scalar type ‘std::function(Nor::Range&)>’ requested
Taking reference from this answer
I tried this way :
std::function<std::vector<uint8_t>(Nor::Range)> f = std::bind(&Car::drive, this);
And Got this error:
/usr/include/c++/9/functional:775:7: error: static assertion failed: Wrong number of arguments for pointer-to-member
774 | static_assert(_Varargs::value
| ~~~~~
775 | ? sizeof...(_BoundArgs) >= _Arity::value + 1
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
776 | : sizeof...(_BoundArgs) == _Arity::value + 1,
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
seminar.cpp: In member function ‘std::vector<unsigned char> Tom::Car::Car::road(Nor::Driving&)’:
seminar.cpp:23:73: error: conversion from ‘std::_Bind_helper (Tom::Car::Car::*)(Nor::Range&), Tom::Car::Car*>::type’ {aka ‘std::_Bind (Tom::Car::Car::*(Tom::Car::Car*))(Nor::Range&)>’} to non-scalar type ‘std::function(Nor::Range)>’ requested
23 | std::function<std::vector<uint8_t>(Nor::Range)> f = std::bind(&Car::drive, this);
| ~~~~~~~~~^~~~~~~~~~~~~~~~~~~
seminar.cpp:25:40: error: ‘Func’ was not declared in this scope
25 | return Content(driving, 1, Func);
See here live
I don’t know correctly what I am doing wrong in moving the implementation of drive function can someone please help with implementing the corrrect way.
Note:: I’m fine if the solution uses another way to pass the function ie by not using std::function . Thanks
>Solution :
In seminar.cpp, here:
std::function<std::vector<uint8_t>(Nor::Range &)> Func = &drive;
drive is a member function. It needs the this pointer to be called.
You can easily solve this by wrapping it in a lambda:
std::function<std::vector<uint8_t>(Nor::Range &)> Func = [this](Nor::Range & r) {
return this->drive(r);
};
If you prefer the std::bind method that you tried, you need a placeholder for the Nor::Range& parameter:
std::function<std::vector<uint8_t>(Nor::Range &)> Func = std::bind(&Car::drive, this, std::placeholders::_1);
Also, you really don’t need the std::function at all since it’s a local variable that you immediately pass to another function, so just use auto instead (or pass it directly without an intermediate variable):
auto Func = [this](Nor::Range & r) {
return this->drive(r);
};