Is it possible to use a lambda function to create an alias to a template class function? Something like this:
#include <iostream>
using namespace std;
int calcDouble(int a) { return a * 2; }
int calcMultiply_10(int a) { return a * 10; }
struct foo
{
template<void (*func)(int)>
int generic(int value)
{
return func(value);
}
static auto double_10 = [this] { generic<calcDouble>(10); };
static auto double_20 = [this] { generic<calcDouble>(20); };
static auto multiply_10_20 = [this] { generic<calcMultiply_10>(20); };
}
int main() {
foo f;
cout << "double_10: " <<f.double_10() << endl;
cout << "double_20: " <<f.double_20() << endl;
return 0;
}
Your particular example doesn't compile, and would be dangerous if it did - capturing this by value means that as soon as the class is copied/moved the captured this will point to a wrong or invalid memory location.
Just use member functions:
auto double_10() { return generic<calcDouble>(10); }
auto double_20() { return generic<calcDouble>(20); }
auto multiply_10_20() { return generic<calcMultiply_10>(20); }
live example on wandbox
Related
I want function f2 in structure node to call function f1 in node instead of global function f1.
#include <iostream>
#include <functional>
int f1()
{
return 1;
}
int f2()
{
return f1();
}
struct node
{
int f1()
{
return 2;
}
std::function<int()> f2 = ::f2;
};
int main()
{
node a;
std::cout << a.f2() << "\n";
return 0;
}
I want function f2 in structure node to call function f1 in node instead of global function f1.
You can achieve this by adding a (second) constructor to node which accepts a callable to assign to node::f2. Something like this, passing in a lambda to that constructor to capture the object itself:
#include <iostream>
#include <functional>
int f1()
{
return 1;
}
int f2()
{
return f1();
}
struct node
{
node () = default;
node (std::function<int()> assign_to_f2) { f2 = assign_to_f2; }
int f1 ()
{
return 2;
}
std::function<int()> f2 = ::f2;
};
int main()
{
node a;
std::cout << a.f2() << "\n";
node b ([&b] () { return b.f1 (); });
std::cout << b.f2() << "\n";
return 0;
}
Output:
1
2
Live Demo
Using a producer-consumer pattern I'd like to submit calls to the member functions of struct A i.e., func_1() and func_2() by passing the name of the function object (something like A::func_1), and a list of arguments taken by these functions in a queue-like buffer queue_in. I'd then like to collect these results (here a std::variant of all the possible return types (double and int)) in another buffer queue_out for later processing.
At the moment, I'm only able to hack my way by having q_in hold std::string and enumerating all the possibilities manually. Of course I'm also missing passing any possible arguments to func_1() and func_2().
#include <iostream>
#include "blockingconcurrentqueue.h" // https://github.com/cameron314/concurrentqueue
#include <variant>
using Result = std::variant<double, int>;
using queue_in = moodycamel::BlockingConcurrentQueue<std::string>;
using queue_out = moodycamel::BlockingConcurrentQueue<Result>;
struct A {
explicit A(queue_in &q_in, queue_out &q_out) {
std::thread t([&]() {
for (;;) {
std::string s;
if (q_in.wait_dequeue_timed(s, -1)) {
if (s == "1")
q_out.enqueue(this->func_1());
else if (s == "2")
q_out.enqueue(this->func_2());
}
std::this_thread::sleep_for(std::chrono::seconds(1));
}
});
t.join();
}
double func_1() const { // func_1 might have arguments!
std::cout << "func_1() called.\n";
return 13.0f;
}
int func_2() const { // func_2 might have arguments!
std::cout << "func_2() called.\n";
return 1;
}
};
void producer(queue_in &q_in, const size_t N) {
for (size_t i = 0; i != N; ++i) {
q_in.enqueue("1" /*..., arguments for func_1*/); // prefer to call the function-object instead
q_in.enqueue("2" /*..., arguments for func_2*/);
}
}
void result_consumer(queue_out &q_out) {
for (;;) {
Result r;
if (q_out.wait_dequeue_timed(r, -1)) {
std::visit([](auto &&arg) {
std::cout << "The result is: " << arg << std::endl;
}, r);
}
std::this_thread::sleep_for(std::chrono::seconds(0));
}
}
int main() {
const size_t N = 2;
queue_in q_in;
queue_out q_out;
std::thread producer_thread(producer, std::ref(q_in), N);
std::thread result_consumer_thread(result_consumer, std::ref(q_out));
A a(q_in, q_out);
producer_thread.join();
result_consumer_thread.join();
}
How can I do this instead via function objects? Second, how do I invoke any possible arguments for the member functions of A. And third, can I avoid the use of a std::variant like result type?
Taking inspiration from #n.1.8e9-where's-my-sharem. 's comments here's how I got the above to work using std::function and std::bind
using Result = std::variant<double, int>;
using FO = std::function<Result(void)>;
using queue_in = moodycamel::BlockingConcurrentQueue<FO>;
using queue_out = moodycamel::BlockingConcurrentQueue<Result>;
struct A {
explicit A(queue_in &q_in, queue_out &q_out) {
std::thread t([&]() {
for (;;) {
FO o;
if (q_in.wait_dequeue_timed(o, -1)) {
q_out.enqueue(o());
}
std::this_thread::sleep_for(std::chrono::seconds(1));
}
});
t.detach();
}
void consumer() {
}
double func_1(double x, const size_t &s) const { // note the arguments
std::cout << "func_1() called.\n";
return x + s;
}
int func_2(int x) const {
std::cout << "func_2() called.\n";
return x;
}
};
void producer(const A& a, queue_in &q_in, const size_t N) { // note the first argument: the producer() must know about some instance of A
for (size_t i = 0; i != N; ++i) {
FO o;
if (i % 2 == 0)
o = std::bind(&A::func_1, a, double(i), size_t(10));
else
o = std::bind(&A::func_2, a, int(i));
q_in.enqueue(o);
}
}
void result_consumer(queue_out &q_out) {
for (;;) {
Result r;
if (q_out.wait_dequeue_timed(r, -1)) {
std::visit([&](auto &&arg) {
std::cout << "The result is: " << arg << std::endl;
}, r);
}
std::this_thread::sleep_for(std::chrono::seconds(1));
}
}
int main() {
const size_t N = 2;
queue_in q_in;
queue_out q_out;
A a(q_in, q_out);
std::thread producer_thread(producer, std::ref(a), std::ref(q_in), N);
std::thread result_consumer_thread(result_consumer, std::ref(q_out));
producer_thread.join();
result_consumer_thread.join();
}
Output:
func_1() called.
The result is: 10
func_2() called.
The result is: 1
I want to be able to return a function from a class, so that I do not need to if-else through a return type.
I have a class that returns multiple strings. Instead, I want to return multiple functions.
#include <iostream>
class Handler
{
private:
public:
int handleMessage(int code)
{
return code+1;
}
};
void func1();
void func2();
void func3();
int main (int argc, char *argv[])
{
Handler handle;
int code = handle.handleMessage(0);
if(code == 1)
{
func1();
}
return 0;
}
void func1(){ std::cout << "1" << std::endl;}
void func2(){ std::cout << "2" << std::endl;}
void func3(){ std::cout << "3" << std::endl;}
What I want is: That the function handleMessage in the class Handler returns something so that in my main application I do not have to use if-else.
So the main looks like this:
function = handle.handleMessage(0);
And the application will choose which function it will run.
for example:
function = handle.handleMessage(0); //will run func1
function = handle.handleMessage(1); //will run func2
You can modify the member function such that it returns a function pointer, e.g.
using fptr = void (*)();
struct Handler
{
fptr handleMessage (int code)
{
if (code == 0)
return &func1;
else if (code == 1)
return &func2;
else
return &func3;
}
};
This can be invoked as follows
Handler handle;
auto f = handle.handleMessage(0);
f();
Note that the above if-else if-else dispatch isn't ideal. Prefer a data member that stores the function pointers and associates them with a code, e.g. using a std::unordered_map.
Note that when you need to return stateful function objects in the future, this approach will fail. Then, you need to embrace std::function which is able to wrap lambdas with closures or custom types with an operator() overload.
There are several ways to do so, the simplest one, you can use an std::function. In this example we returning a lambda function for each case. You can replace it with the functions you just wrote.
class Handler {
public:
std::function<void()> handleMessage(int code) {
code = code + 1; // ++code or whatever
if (code == X) {
return []() { std::cout << "Cool! I'am x!" << std::endl; };
} else if (code == Y) {
return []() { std::cout << "Cool! I'am x!" << std::endl; };
} else if (...) {
...
} else {
....
}
}
};
Then your main function becomes:
int main (int argc, char *argv[]) {
Handler handle;
const auto func = handle.handleMessage(0);
func();
return 0;
}
You can replace the swith/if case statement by an array storing the different functions, like they mentioned in the comments.
If you dont want to pay the extra virtual function call regarding the usage of an std::function, you can use an alias like the answer below or just the auto keyword:
class Handler {
public:
constexpr auto handleMessage(int code) {
code = code + 1; // ++code or whatever
if (code == X) {
return &func1;
} else if (code == Y) {
return &func2;
} else if (...) {
...
} else {
....
}
}
};
std::function is a powerful tool. The tiny brother is a simple function pointer.
I transformed MCVE respectively to return a function pointer:
#include <iostream>
typedef void (*FuncPtr)();
void func1();
void func2();
void func3();
void funcError();
class Handler
{
private:
public:
FuncPtr handleMessage(int code)
{
switch (code + 1) {
case 1: return &func1;
case 2: return &func2;
case 3: return &func3;
default: return &funcError;
}
}
};
int main (int argc, char *argv[])
{
Handler handle;
FuncPtr pFunc = handle.handleMessage(0);
pFunc();
return 0;
}
void func1(){ std::cout << "1" << std::endl;}
void func2(){ std::cout << "2" << std::endl;}
void func3(){ std::cout << "3" << std::endl;}
void funcError(){ std::cout << "ERROR!" << std::endl;}
Output:
1
Live Demo on coliru
You can return a function with return_type(*function_name)(argument_type1, argument_type2...) so a function that looks like:
double f(int a, int b);
has the name double(*f)(int, int).
Worth mentioning is C++11's std::function which requires the <functional> header. It has a more intuitive usage: std::function<double(int, int)> but also adds a bit of overhead.
I would also like to suggest the usage of C++17's std::optional as for the case when the variable code goes out of bounds. This implementation requires the <optional> header.
std::optional<void(*)()> handleMessage(int code){
switch (code) {
case 0: return std::optional(func1);
case 1: return std::optional(func2);
case 2: return std::optional(func3);
}
return std::nullopt; //empty
}
usage in main looks like the following:
Handler handle;
auto func = handle.handleMessage(0);
if (func.has_value()) {
func.value()();
}
as this allows to check if func.has_value() which is quite convenient.
Use an array of functions.
void func1(){ std::cout << "1" << std::endl; }
void func2(){ std::cout << "2" << std::endl; }
void func3(){ std::cout << "3" << std::endl; }
typedef void (* func ) () ;
class Handler {
public:
func handleMessage(int code)const{
static const func F[] = { func1, func2, func3 };
return F[ code ];
}
};
int main()
{
Handler handler;
func f = handler.handleMessage(0); // returns func1
f();
}
live example
you can map the ints to a function or lambda, but read befor what at() does and what happens if the key is not found!!
void function1()
{
std::cout << "q1" << std::endl;
}
void function2()
{
std::cout << "q2" << std::endl;
}
int main(int argc, char* argv[])
{
std::map<int, std::function<void(void)>> map;
map.insert(std::make_pair(1, function1));
map.insert(std::make_pair(1, function2));
map.at(1)();
I would like to offer solution without any if-else block. You just need to templatize your Handler::handleMessage function. Something like this:
// Class declaration
class Handler
{
private:
public:
template<int code>
void handleMessage();
};
and specialize the function template for particular codes:
// Function template specializations.
template<>
void Handler::handleMessage<1>()
{
std::cout << "1" << std::endl;
}
template<>
void Handler::handleMessage<2>()
{
std::cout << "2" << std::endl;;
}
template<>
void Handler::handleMessage<3>()
{
std::cout << "3" << std::endl;;
}
// All cases, except 1, 2 and 3
template<int code>
void Handler::handleMessage()
{
std::cout << "Anything else" << std::endl;;
}
The usage may look like:
Handler h;
h.handleMessage<1>(); // Prints 1
h.handleMessage<2>(); // Prints 2
h.handleMessage<3>(); // Prints 3
h.handleMessage<323>(); // Prints 'Anything else'
So I'm trying to implement function parameters which can be uninitialized. Here is the code which I have written. My question is if it's legal by the ISO C++ standard (version 14 if possible).
#include <iostream>
#include <typeinfo>
using namespace std;
template<typename type>
struct nzeroinittmpliteral
{
nzeroinittmpliteral() { }
nzeroinittmpliteral(type arg) { d = arg; }
//nzeroinittmpliteral(const nzeroinittmpliteral &) = delete;
operator type () & { return d; }
operator type () && { return d; }
type d;
} ;
void func(bool bIsPointerValid, nzeroinittmpliteral<int *> pVar = {})
{
if(bIsPointerValid)
{
cout << *pVar << endl;
}
else
{
pVar = new int;
*pVar = 8;
cout << *pVar << endl;
delete pVar;
}
}
int main()
{
func(true, { (int *)&(const int &)int{9} } );
func(false);
}
If you want to pass a parameter that may be uninitialized, simply don't pass it, use overloading. Look:
void func(int value)
{
cout << value << endl;
}
void func()
{
// no 'value' was initialized here :)
func(8);
}
Or simply give a default value to the parameter if you will provide one anyway in your body:
void func(int value = 8)
{
cout << value << endl;
}
Besides that, you can take a look at boost::optional:
void func(boost::optional<int> optvalue = boost::none) {
if (optvalue) {
cout << *optvalue << endl;
} else {
// nothing passed
cout << "foo" << endl;
}
}
Directly answering your question: your code is valid.
func(true, { (int *)&(const int &)int{9} } );
By casting the temporary to a const reference, you extend its lifetime to the lifetime of the reference itself, which ends after func returns. But this is too redundant, you could simply have written:
void func(int* value) { if (value) {...} }
func(&(const int &)9);
func(nullptr);
The actual parameter being passed is your nzeroinittmpliteral and it is initialized by calling one of the constructors, always. The default constructor doesn't initialize the d member, but this is no big improvement as it is just a pointer. Using nullptr is better and removes the need for the bool parameter.
class Foo {
public:
Foo() { do_something = &Foo::func_x; }
int (Foo::*do_something)(int); // function pointer to class member function
void setFunc(bool e) { do_something = e ? &Foo::func_x : &Foo::func_y; }
private:
int func_x(int m) { return m *= 5; }
int func_y(int n) { return n *= 6; }
};
int
main()
{
Foo f;
f.setFunc(false);
return (f.*do_something)(5); // <- Not ok. Compile error.
}
How can I get this to work?
class A{
public:
typedef int (A::*method)();
method p;
A(){
p = &A::foo;
(this->*p)(); // <- trick 1, inner call
}
int foo(){
printf("foo\n");
return 0;
}
};
void main()
{
A a;
(a.*a.p)(); // <- trick 2, outer call
}
The line you want is
return (f.*f.do_something)(5);
(That compiles -- I've tried it)
"*f.do_something" refers to the pointer itself --- "f" tells us where to get the do_something value from. But we still need to give an object that will be the this pointer when we call the function. That's why we need the "f." prefix.
class A {
int var;
int var2;
public:
void setVar(int v);
int getVar();
void setVar2(int v);
int getVar2();
typedef int (A::*_fVar)();
_fVar fvar;
void setFvar(_fVar afvar) { fvar = afvar; }
void insideCall() { (this->*fvar)(); }
};
void A::setVar(int v)
{
var = v;
}
int A::getVar()
{
std::cout << "A::getVar() is called. var = " << var << std::endl;
return var;
}
void A::setVar2(int v2)
{
var2 = v2;
}
int A::getVar2()
{
std::cout << "A::getVar2() is called. var2 = " << var2 << std::endl;
return var2;
}
int main()
{
A a;
a.setVar(3);
a.setVar2(5);
// a.fvar = &A::getVar;
a.setFvar(&A::getVar);
(a.*a.fvar)();
a.setFvar(&A::getVar2);
(a.*a.fvar)();
a.setFvar(&A::getVar);
a.insideCall();
a.setFvar(&A::getVar2);
a.insideCall();
return 0;
}
I extended Nick Dandoulakis's answer. Thank you.
I added a function which set the member function pointer from outside of the class. I added another function which can be called from outside to show inner call of member function pointer.
Try (f.*do_something)(5);
#include<iostream>
using namespace std;
class A {
public:
void hello()
{
cout << "hello" << endl;
};
int x = 0;
};
void main(void)
{
//pointer
A * a = new A;
void(A::*pfun)() = &A::hello;
int A::*v1 = &A::x;
(a->*pfun)();
a->*v1 = 100;
cout << a->*v1 << endl << endl;
//-----------------------------
A b;
void(A::*fun)() = &A::hello;
int A::*v2 = &A::x;
(b.*fun)();
b.*v2 = 200;
cout << b.*v2 << endl;
}
I think calling a non static member of the class could also be done using a static member function.