This question already has answers here:
Invoke a c++ class method without a class instance?
(5 answers)
Closed 1 year ago.
I have created a function within a class. How do I call that function in int main() without declaring any object for the class?
You can do it by using a static member function as shown below:
#include <iostream>
class NAME
{
public:
//define a static member function
static void print_st()
{
std::cout<<"static print_st callled"<<std::endl;
}
};
int main()
{
//call static member function without using an object
NAME::print_st();
return 0;
}
What you can do is to write your function inside the class as static. This lets you call the function without declaring an Object
Related
This question already has answers here:
How can I pass a member function where a free function is expected?
(9 answers)
how to pass a non static-member function as a callback?
(8 answers)
Pass Member Function as Parameter to other Member Function (C++ 11 <function>) [duplicate]
(2 answers)
Closed 4 months ago.
I need to change the prototype of a function pointer of a class. So, I was hoping inheriting it and doing the following would work, but it doesn't ("invalid use of non-static member function 'void B::myIntCallback(unsigned int)"):
class A {
public:
typedef void (*intCallback_t)(unsigned int myInt);
A(intCallback_t intCallback) {}
};
class B : A {
public:
typedef void (*charCallback_t)(unsigned char myChar);
B(charCallback_t charCallback) : A(this->myIntCallback) {
charCallback_ = charCallback;
}
private:
charCallback_t charCallback_;
void myIntCallback(unsigned int myInt) {
charCallback_((unsigned char)myInt);
}
};
Does anybody know how I can solve this? I can't change class A.
You are trying to pass a member function (B::myIntCallback) to a function pointer argument. Since the member function needs an object to be called on, his would require some kind of capturing, e.g. a lambda capturing this or an std::bind expression. Unfortunately, neither is possible with a plain function pointer, see also Passing capturing lambda as function pointer.
If possible, you may want to consider changing the class A to accept either a std::function or a template argument as the type of the callback. See also Should I use std::function or a function pointer in C++?.
This question already has answers here:
C++ Call Pointer To Member Function
(4 answers)
Calling C++ member functions via a function pointer
(10 answers)
Closed 9 months ago.
Consider this class:
class Downloader {
private:
bool video_or_audio;
// other variables [...]
// [...]
void downloadVideo(std::string videoURL);
void downloadAudio(std::string audioURL);
public:
void download();
}
Now, download() is defined this way:
void Downloader::download(){
std::ifstream url_list;
void (*download_func)(std::string) = video_or_audio == 0 ? downloadVideo : downloadAudio; // Compiler says here: "Reference to non static member function must be called".
if(video_or_audio == 0){
url_list.open("video_list.txt");
}
else{
url_list.open("audio_list.txt");
}
std::string url;
while(std::getline(url_list, url)){
download_func(url); // Calling the function pointed by the pointer defined in line 2 of the function download().
}
}
My compiler (clang) says: "Reference to non static member function must be called" in the second line of function download() definition. Why is this happening and how can I solve this problem?
A solution appears to be defining downloadVideo() and downloadAudio() functions to be static in the class declaration. However, if I do so, I cannot access private variables members of class Downloader, that's not desirable, as I need these variables.
Thank you!
Once calling a member function by pointer, you need to provide two things:
Member function itself
Address of particular instance of given class at which the member function to be called (because the function can access class members thus you have to advise which of the instances is the right one)
void (Downloader::*download_func)(std::string) = video_or_audio == 0 ? &Downloader::downloadVideo : &Downloader::downloadAudio; // Correct the signatures.
...
while(std::getline(url_list, url)){
(this->*download_func)(url);
}
So, here we changed download_func from "just function pointer" to a member function pointer. Then, later in the loop body, we call this member function on this instance (however you can pass an instance as a param if necessary).
This question already has answers here:
Undefined reference to static class member
(9 answers)
How to initialize private static members in C++?
(18 answers)
Closed 6 years ago.
I have a file A.hpp as such:
class A
{
private:
static std::string s;
public:
void modify_string();
};
I am implementing this in a file A.cpp as such:
#include "A.hpp"
void A::modify_string()
{
s = "something"; // Error here.
}
My main class:
int main()
{
A a;
a.modify_string();
}
I understand static variables are shared by all the class instances. I also went through this SO post where it says how to access the static member. Public static member of class . Could you please let me know where my concept is missing at?
Edit:
I am getting this error:
error: undefined reference to A::s
When you define:
void modify_string() {
s = "something"; // Error here.
}
You are creating a new function, not defining the member function modify_string of the class A. You need to do:
void A::modify_string() {
To inform the compiler that you are defining the member function modify_string for class A.
You also need a ; after your class definition.
Finally, the variable s is static so it needs to be defined seperatly somewhere so the linker can find a reference to it. So add:
std::string A::s = "default";
This was clearly described in the link you provided for your question.
Here is a working example: http://ideone.com/iQ6Kux
You need to reserve storage for s in exactly one compilation unit.
Do that by writing
std::string A::s;
In exactly one source file.
Your definition void modify_string() {...} in A.cpp is not defining the member function of the class, it's defining a separate global function with the same name. You probably meant
void A::modify_string()
{
s = "something";
}
This question already has answers here:
How can I test private members and methods of classes?
(8 answers)
Closed 8 years ago.
I have a class A having a number of Private/Public member functions/variables. I have added a new private member function to it. As part of a testing the newly added method, I need to invoke the private member function from my test class. In the case of member variables I have seen an example like below:
#include <iostream>
using namespace std;
class test
{
private:
int myInt;
public:
int getInt () {return myInt;}
};
int main ()
{
test t;
int* p = (int*) & t;
*p = 20;
cout << t.getInt ();
}
Output:20
Is there any reliable way I can access the private member function such that there is no code modification in the class A?
You can change class access modifiers with macro #define private public for testing, but it isn't good idea. I think you need to review test approach
This question already has answers here:
Undefined reference to static variable [duplicate]
(2 answers)
Closed 9 years ago.
I am beginner to C++ and have a doubt about static member variables and member functions.
I have implemented a class as follows -
class Foo
{
private:
static int myVariable;
public:
static void setMyVariable()
{
myVariable = 100;
}
static void resetMyVariable()
{
myVariable = 0;
}
};
There are following considerations when I wrote a code like that -
I want only one instance of class Foo. Thats why I made all member variables and functions as static.
I don't want the outside code to touch myVariable
I have put this class in a header file and included in my main file. When I do this, I get an error undefined reference to Foo::myVariable
I want to know if I can write a code which can satisfy above requirements?
Thanks !
You need to define static class variables somewhere:
e.g. in your main C++ file,
int Foo::myVariable;
Note that technically, by making everything static, you may have no instances of Foo.