I am trying to generalize my benchmarking function, by having it receive the function to benchmark as the first parameter and the number of iterations as the second.
But since the function to benchmark needs to receive additional parameters, I thought I would fill in the parameters in the body of a lambda function and pass that to the benchmarking function. (I think that is called currying?)
Anyway I can not get it to compile:
main.cpp:43:62: error: invalid initialization of non-const reference of type ‘std::function<double*()>&’ from an rvalue of type ‘main(int, char**)::<lambda()>’
bench::bench([=](){rng::gpu_r_exp((int) 10e6, lambda);}, 50);
The function declaration looks like this:
void bench(std::function<double*()>& funct_to_bench, int repeats);
and I use it like this:
bench::bench([=](){rng::gpu_r_exp((int) 10e6, lambda);}, 50);
Since the compiler bickers about non-const again, I should maybe add, that gpu_r_exp utilizes a global variable which stores the rngState (it also did not like non-const parameters in gpu_r_exp).
I am really stuck. I just want to fill in the parameters and pass the pre-prepared function handle to the benchmarking function, so that it can wrap a timer with a progress bar around it.
EDIT: I should add that the parameter called lambda is a double, which is the parameter of the exponential distribution and has nothing to do with the lambda function.
Given that the benchmark wrapper is small, it doesn't make sense to worry about what's passed in, or whether it can be converted to std::function. Just take what comes and as long as it can be called, you're golden:
template <typename Fun>
void benchmarkCallable(size_t iterations, Fun &&callable)
{
//...
while (iterations--)
callable();
//..
}
If you worry that the //... sections are getting unwieldy, you can factor them out into a class:
class ScopedBenchmark {
// start time, other state needed, etc.
public:
ScopedBenchmark() { /* capture initial state */ }
~ScopedBenchmark() { /* dump results etc */ }
};
template <typename Fun>
void benchmarkCallable(size_t iterations, Fun &&callable)
{
ScopedBenchmark benchmark;
while (iterations--)
callable();
}
int main()
{
benchmarkCallable(1'000'000, []{ printf("foo!\n"); });
}
Related
The Goal:
decide during runtime which templated function to use and then use it later without needing the type information.
A Partial Solution:
for functions where the parameter itself is not templated we can do:
int (*func_ptr)(void*) = &my_templated_func<type_a,type_b>;
this line of code can be modified for use in an if statement with different types for type_a and type_b thus giving us a templated function whose types are determined during runtime:
int (*func_ptr)(void*) = NULL;
if (/* case 1*/)
func_ptr = &my_templated_func<int, float>;
else
func_ptr = &my_templated_func<float, float>;
The Remaining Problem:
How do I do this when the parameter is a templated pointer?
for example, this is something along the lines of what I would like to do:
int (*func_ptr)(templated_struct<type_a,type_b>*); // This won't work cause I don't know type_a or type_b yet
if (/* case 1 */) {
func_ptr = &my_templated_func<int,float>;
arg = calloc(sizeof(templated_struct<int,float>, 1);
}
else {
func_ptr = &my_templated_func<float,float>;
arg = calloc(sizeof(templated_struct<float,float>, 1);
}
func_ptr(arg);
except I would like type_a, and type_b to be determined during runtime. I see to parts to the problem.
What is the function pointers type?
How do I call this function?
I think I have the answer for (2): simply cast the parameter to void* and the template function should do an implicit cast using the function definition (lease correct me if this won't work as I think it will).
(1) is where I am getting stuck since the function pointer must include the parameter types. This is different from the partial solution because for the function pointer definition we were able to "ignore" the template aspect of the function since all we really need is the address of the function.
Alternatively there might be a much better way to accomplish my goal and if so I am all ears.
Thanks to the answer by #Jeffrey I was able to come up with this short example of what I am trying to accomplish:
template <typename A, typename B>
struct args_st {
A argA;
B argB;
}
template<typename A, typename B>
void f(struct args_st<A,B> *args) {}
template<typename A, typename B>
void g(struct args_st<A,B> *args) {}
int someFunction() {
void *args;
// someType needs to know that an args_st struct is going to be passed
// in but doesn't need to know the type of A or B those are compiled
// into the function and with this code, A and B are guaranteed to match
// between the function and argument.
someType func_ptr;
if (/* some runtime condition */) {
args = calloc(sizeof(struct args_st<int,float>), 1);
f((struct args_st<int,float> *) args); // this works
func_ptr = &g<int,float>; // func_ptr should know that it takes an argument of struct args_st<int,float>
}
else {
args = calloc(sizeof(struct args_st<float,float>), 1);
f((struct args_st<float,float> *) args); // this also works
func_ptr = &g<float,float>; // func_ptr should know that it takes an argument of struct args_st<float,float>
}
/* other code that does stuff with args */
// note that I could do another if statement here to decide which
// version of g to use (like I did for f) I am just trying to figure out
// a way to avoid that because the if statement could have a lot of
// different cases similarly I would like to be able to just write one
// line of code that calls f because that could eliminate many lines of
// (sort of) duplicate code
func_ptr(args);
return 0; // Arbitrary value
}
Can't you use a std::function, and use lambdas to capture everything you need? It doesn't appear that your functions take parameters, so this would work.
ie
std::function<void()> callIt;
if(/*case 1*/)
{
callIt = [](){ myTemplatedFunction<int, int>(); }
}
else
{
callIt = []() {myTemplatedFunction<float, float>(); }
}
callIt();
If I understand correctly, What you want to do boils down to:
template<typename T>
void f(T)
{
}
int somewhere()
{
someType func_ptr;
int arg = 0;
if (/* something known at runtime */)
{
func_ptr = &f<float>;
}
else
{
func_ptr = &f<int>;
}
func_ptr(arg);
}
You cannot do that in C++. C++ is statically typed, the template types are all resolved at compile time. If a construct allowed you to do this, the compiler could not know which templates must be instanciated with which types.
The alternatives are:
inheritance for runtime polymorphism
C-style void* everywhere if you want to deal yourself with the underlying types
Edit:
Reading the edited question:
func_ptr should know that it takes an argument of struct args_st<float,float>
func_ptr should know that it takes an argument of struct args_st<int,float>
Those are incompatible. The way this is done in C++ is by typing func_ptr accordingly to the types it takes. It cannot be both/all/any.
If there existed a type for func_ptr so that it could take arguments of arbitrary types, then you could pass it around between functions and compilation units and your language would suddenly not be statically typed. You'd end up with Python ;-p
Maybe you want something like this:
#include <iostream>
template <typename T>
void foo(const T& t) {
std::cout << "foo";
}
template <typename T>
void bar(const T& t) {
std::cout << "bar";
}
template <typename T>
using f_ptr = void (*)(const T&);
int main() {
f_ptr<int> a = &bar<int>;
f_ptr<double> b = &foo<double>;
a(1);
b(4.2);
}
Functions taking different parameters are of different type, hence you cannot have a f_ptr<int> point to bar<double>. Otherwise, functions you get from instantiating a function template can be stored in function pointers just like other functions, eg you can have a f_ptr<int> holding either &foo<int> or &bar<int>.
Disclaimer: I have already provided an answer that directly addresses the question. In this answer, I would like to side-step the question and render it moot.
As a rule of thumb, the following code structure is an inferior design in most procedural languages (not just C++).
if ( conditionA ) {
// Do task 1A
}
else {
// Do task 1B
}
// Do common tasks
if ( conditionA ) {
// Do task 2A
}
else {
// Do task 2B
}
You seem to have recognized the drawbacks in this design, as you are trying to eliminate the need for a second if-else in someFunction(). However, your solution is not as clean as it could be.
It is usually better (for code readability and maintainability) to move the common tasks to a separate function, rather than trying to do everything in one function. This gives a code structure more like the following, where the common tasks have been moved to the function foo().
if ( conditionA ) {
// Do task 1A
foo( /* arguments might be needed */ );
// Do task 2A
}
else {
// Do task 1B
foo( /* arguments might be needed */ );
// Do task 2B
}
As a demonstration of the utility of this rule of thumb, let's apply it to someFunction(). ... and eliminate the need for dynamic memory allocation ... and a bit of cleanup ... unfortunately, addressing that nasty void* is out-of-scope ... I'll leave it up to the reader to evaluate the end result. The one feature I will point out is that there is no longer a reason to consider storing a "generic templated function pointer", rendering the asked question moot.
// Ideally, the parameter's type would not be `void*`.
// I leave that for a future refinement.
void foo(void * args) {
/* other code that does stuff with args */
}
int someFunction(bool condition) {
if (/* some runtime condition */) {
args_st<int,float> args;
foo(&args);
f(&args); // Next step: pass by reference instead of passing a pointer
}
else {
args_st<float,float> args;
foo(&args);
f(&args); // Next step: pass by reference instead of passing a pointer
}
return 0;
}
Your choice of manual memory management and over-use of the keyword struct suggests you come from a C background and have not yet really converted to C++ programming. As a result, there are many areas for improvement, and you might find that your current approach should be tossed. However, that is a future step. There is a learning process involved, and incremental improvements to your current code is one way to get there.
First, I'd like to get rid of the C-style memory management. Most of the time, using calloc in C++ code is wrong. Let's replace the raw pointer with a smart pointer. A shared_ptr looks like it will help the process along.
// Instead of a raw pointer to void, use a smart pointer to void.
std::shared_ptr<void> args;
// Use C++ memory management, not calloc.
args = std::make_shared<args_st<int,float>>();
// or
args = std::make_shared<args_st<float,float>>();
This is still not great, as it still uses a pointer to void, which is rarely needed in C++ code unless interfacing with a library written in C. It is, though, an improvement. One side effect of using a pointer to void is the need for casts to get back to the original type. This should be avoided. I can address this in your code by defining correctly-typed variables inside the if statement. The args variable will still be used to hold your pointer once the correctly-typed variables go out of scope.
More improvements along this vein can come later.
The key improvement I would make is to use the functional std::function instead of a function pointer. A std::function is a generalization of a function pointer, able to do more albeit with more overhead. The overhead is warranted here in the interest of robust code.
An advantage of std::function is that the parameter to g() does not need to be known by the code that invokes the std::function. The old style of doing this was std::bind, but lambdas provide a more readable approach. Not only do you not have to worry about the type of args when it comes time to call your function, you don't even need to worry about args.
int someFunction() {
// Use a smart pointer so you do not have to worry about releasing the memory.
std::shared_ptr<void> args;
// Use a functional as a more convenient alternative to a function pointer.
// Note the lack of parameters (nothing inside the parentheses).
std::function<void()> func;
if ( /* some runtime condition */ ) {
// Start with a pointer to something other than void.
auto real_args = std::make_shared<args_st<int,float>>();
// An immediate function call:
f(real_args.get());
// Choosing a function to be called later:
// Note that this captures a pointer to the data, not a copy of the data.
// Hence changes to the data will be reflected when this is invoked.
func = [real_args]() { g(real_args.get()); };
// It's only here, as real_args is about to go out of scope, where
// we lose the type information.
args = real_args;
}
else {
// Similar to the above, so I'll reduce the commentary.
auto real_args = std::make_shared<args_st<float,float>>();
func = [real_args]() { g(real_args.get()); };
args = real_args;
}
/* other code that does stuff with args */
/* This code is probably poor C++ style, but that can be addressed later. */
// Invoke the function.
func();
return 0;
}
Your next step probably should be to do some reading on these features so you understand what this code does. Then you should be in a better position to leverage the power of C++.
So I wanted to challenge myself by writing a small threadpool in C++, and I wanted to try to mimic the easy to use way that std::thread work with, that you can just create a thread and as parameters send a function and parameters for that function, compared to something like pthreads which force you to have a void* as the only indata for the function.
So far I have been able to use templates and parameter packs to create a function that can take another function and parameters for it and execute it, but I can't find a way to store them so that I can execute them at a later time (when there is a free thread in the threadpool). I have tried using both std::function together with std::tuple, and std::bind, but since I don't know exactly what types I am dealing with I can't find a way to store the function and the parameters so that I can use them later on in another part of my code, since at that point I no longer know what types everything is of. Down below is some code I have been messing around with that might help show how I mean.
template<typename Function, typename... Arguments>
void TestFunction(Function func, Arguments... parameters)
{
std::function<std::result_of<Function(Arguments...)>::type(Arguments...)>* tempFunc;
tempFunc = new std::function<std::result_of<Function(Arguments...)>::type(Arguments...)>(func);
void* funcPtr = tempFunc;
std::tuple<Arguments...>* tempTuple;
tempTuple = new std::tuple<Arguments...>(parameters...);
void* tuplePtr = tempTuple;
//func(parameters...);
(Arguments...)>*)funcPtr, *(std::tuple<Arguments...>*)tuplePtr);
auto bindTest = std::bind(func, parameters...);
bindTest();
void* bindPtr = &bindTest;
}
int main()
{
TestFunction(std::printf, "%d, %d, %d\n", 3, 2, 1);
getchar();
return 0;
}
It might be that it's not possible to do what I want to do, and in that case I guess I'll just have to switch to an approach more like pthreads. But if anyone knows a work around I would be grateful.
The key thing is that you can store the return type of std::bind in a std::function. Because std::bind returns an object that is callable. You should then be able to store the std::function instance depending on how you want to handle the return type.
template<typename Function, typename... Arguments>
void TestFunction(Function func, Arguments... parameters)
{
using Ret = typename std::result_of<Function>::type;
std::function<Ret()> val{std::bind(func, parameters...)};
}
If you do this when you first recive the function you no longer have to think about the arguments type, and only the return type. How you handle the return type will depend on the usecase of storing the function. One simple approach is to require that Function is a void function, which may make sense if there is no way to pass the value back to the consumer of the API.
I want to call the following function and pass it a function with a parameter. The purpose of that is that it should call the function with my specified parameter so I know what triggered the function (in that case a gpio pin on the Raspberry Pi).
int wiringPiISR( int pin, int edgeType, void (*function)( void ) );
Currently I have:
for ( int i = 0; i < myValues.size(); ++i )
{
int myValue = myValues[ i ];
wiringPiISR( myValue, INT_EDGE_RISING, &myCallback( myValue ) );
}
Though this is giving me the following error:
error: lvalue required as unary ‘&’ operand
Which I can't really understand as to my understanding, myValue is an lvalue or is it not?
Is it what I want do even possible? If so how?
The function wiringPiISR is from a library called wiringPi and I would like to avoid modifying it as much as possible.
You could combine the answers from imreal and Ryan Haining something like this.
std::function<void()> cbfunc;
void myCallback()
{
cbfunc();
}
void myWiringPiISR(int val, int mask, std::function<void()> callback)
{
cbfunc = callback;
wiringPiISR(val, mask, &myCallback);
}
... and then use it...
void myActualCallback(int v)
{
... do something...
}
myWiringPiISR(myValue, INT_EDGE_RISING, std::bind(myActualCallback, myValue));
No need to patch library, and you can use all the bind/function goodness. I'll leave you to find a way around the thread safety issues...
How does it work? Put simply 'std::bind' is binding together a function and it's parameters into a single std:function object which can then be 'called' from the myCallback function which acts as a shim around the callback that you pass. I'd given the callback function a confusing name before, but this edit has hopefully fixed that.
You can "vomit" the function. This doesn't require a user-defined mutable global variable and is thread-safe, unless you have a compiler that supports multiple threads but not per-thread exceptions which would be basically unusable.
myWiringPiISRWrapper(Value value, int edge, std::function<void()> func) {
try {
throw &func;
} catch(...) {
myWiringPiISR(value, edge, [] {
try {
throw;
} catch(std::function<void()>* func) {
(*func)();
}
});
}
}
It's disgusting and slow, but it's totally encapsulated which I think is a worthwhile upside. Note that this only works if the callback is never executed after the call to myWiringPiISR returns. In this case you can of course have a callback with whatever bound state you desire.
If myValue is something you can decide at compile time, you could set it statically and use an intermediate function to pass in.
void myCallbackHelper() {
static constexpr int myValue = 3;
myCallback(myValue);
}
wiringPiISR(myValue, INT_EDGE_RISING, &myCallbackHelper);
If you need to determine myValue at run time, you could still accomplish this, but not really thread-safely.
int& getMyValue() {
static int myValue;
return myValue;
}
void setMyValue(int i) {
getMyValue() = i;
}
void myCallbackHelper() {
myCallback(getMyValue());
}
Then set it and call
setMyValue(3);
wiringPiISR(myValue, INT_EDGE_RISING, &myCallbackHelper);
I looked up wiringPiISR and found that it is some sort of api call, so i am assuming you cannot change it.
Having said that, there is a reason most api-calls with a function-pointer-callback look sort of like this
void setCallback( void (*function)(void* data), void* userdata);
This allows people to cast their struct {blabla} data; to add some userdata, and when the function is called, it is passed along.
So basically, apart from hacking stuff with static variables, you can't pass any arguments.
You need to use std::function and std::bind.
Change your function signature to
int wiringPiISR (int pin, int edgeType, std::function<void()> func);
Inside you can call the callback simply using func()
And your call to:
int myValue = 3;
wiringPiISR(myValue, INT_EDGE_RISING, std::bind(myCallback, myValue));
What this does is create a std::function object (i.e. a callable) that wraps your function and keeps your desired value in its state.
This will only work on C++11 and newer.
If you have c++11, I suggest using std::function - it's quite a bit cleaner.
If not, your function signature is wrong. You want a callback with the type void(int) but your function takes a void()
Say that you define a callback function as such:
typedef std::function<void(float)> Callback;
And you have a function as such:
void ImAFunction(float a)
{
//Do something with a
}
Is there a way to be able to store a function without an argument then pass one to it at a later time?
Such as this:
//Define the Callback storage
Callback storage;
storage = std::bind(ImAFunction, this);
//Do some things
storage(5);
This wont work which I explain with some of my real code below.
I can get close to what I wan't if I bind the value in with the std::bind function. Such as:
//Change
//storage = std::bind(ImAFunction, this);
storage = std::bind(ImAFunction, this, 5.0); //5.0 is a float passed
This works but when I go to pass a value through the function the outcome is whatever I set it to before:
storage(100); //Output is still 5
I am basing the fact that I think this is possible on this article.
http://www.cprogramming.com/tutorial/function-pointers.html
It doesn't use the function or bind functions but it does pass pointer arguments and performs exactly what I need. The reason I don't just skip the bind function is because I am trying to store the function in a class (private) and I can't store it if it's a template because it's created with the class.
The error produced above comes from this code:
struct BindInfo {
Callback keyCallback;
int bindType;
bool isDown;
bool held;
std::string name;
};
template <class T1>
void bindEvent(int bindType, T1* keydownObj, void(T1::*keydownF)(float), std::string name)
{
BindInfo newKeyInfo = { std::bind(keydownF, keydownObj), bindType, false, false, name };
inputBindings.insert(std::pair<int, BindInfo>(BIND_NULL, newKeyInfo));
};
The error is:
No viable conversion from '__bind<void(Main::*&)(float), Main *&>' to 'Callback' (aka 'function<void (float)>'
Is this possible? Thanks in advance.
You can include a placeholder for an unbound argument:
std::bind(&Main::ImAFunction, this, std::placeholders::_1);
If you find that a bit of a mouthful, a lambda might be more readable:
[this](float a){ImAFunction(a);}
It sounds like what you're looking for is a function pointer. While I don't have a lot of experience using them in C++ I have used them in C so: Yes, it is possible. Perhaps something like this:
void (*IAmAFunctionPointer)(float) = &IAmAFunction;
The best way to think about that line is, that IAmAFunctionPointer is a pointer (hence the *), it returns a void, and takes a float. Then later:
float a = 5;
IAmAFunctionPointer(a);
You could even design it so that the callback function is passed into the method (I assume this is what you're looking for).
void DoStuffThenCallback(float a, void (*callback)(float))
{
//DoStuff
callback(a);
}
further reading: http://www.cprogramming.com/tutorial/function-pointers.html
I was thinking about doing a take_timing function that would take the timing of any function passed to it. By any function it means that the arguments this callback takes is unknown by the caller. It would take too the arguments for the callback. But as it doesn't know how to call it, it would take another callback function, the caller of the callback, written by the function user. The stub would be something like this:
void take_timing(
void (*callback)(),
void (*caller(void (*callback)(),void* args_struct),
void* args_struct
)
{
// Start timer
caller(callback,args_struct);
// Stop timer, read timings, record...
}
void some_caller(void (*callback)(),void* args_struct)
{
// Cast "callback" to function signature
// Cast args_struct to some struct with args
// Call the callback with correct args signature
}
So comes my questions:
Is it possible?
Can it use variable arguments list to make it simples? How? I am helpless on this...
Is there a better way of doing it? Or is it just better to do a take_timing for every specific case?
Is there a OOP Design Pattern for this for use with C++?
I myself gave up on this, but put it here out of curiosity, maybe some very useful insights.
Just take a functor as a template argument. Something like:
template<typename F>
nanoseconds take_timing(F f) {
auto start = high_resolution_clock::now();
f();
auto end = high_resolution_clock::now();
return end - start;
}
long long factorial(int i);
take_timing( [](){factorial(20);} ); // wrap call to function taking arguments in zero-argument lambda
You're looking for varargs support, which does exist in ANSI C. A gazillion or so google hits will result if you search for it. Here's one at random: http://www.eskimo.com/~scs/cclass/int/sx11b.html
One way to handle this is to take a boost::bind function object as your parameter:
http://www.boost.org/doc/libs/1_48_0/libs/bind/bind.html
You could hide the fact that closures are involved with macros.