C++ :: Passing Lambdas As Template Parameter
Oct 6, 2013
I've been playing around with this piece of code:
#include <iostream>
#include <string>
#include <algorithm>
template <void(*funky)(const std::string&)>
void callback()
{funky("Hello World!");}
[Code] ....
But when I try to build it, I get this error on line 24:could not convert template argument 'lambda' to 'void (*)(const string&) {aka void (*)(const std::basic_string<char>&)}'|
I thought the lambda expression I wrote would decay to a function pointer matching the template parameter. I can guess that the constexpr qualifier might have changed the type, but without it my compiler complains that lambda needs to be declared as constexpr...
So is there a way to pass lambda expressions as template parameters?
Without having to use std::function
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Dec 26, 2013
Pseudocode:
template<typename T /*, some parameter for member_function */>
class Foo {
public:
void someFunction() {
T t;
t.member_fuction(...);
} }
I'm trying to make the call to T::member_function a templated value because member_function might vary by name in my scenario. Since std::mem_fn isn't a 'type', i can't do something like Foo<std::string, std::mem_fn(&std::string::clear)> foo;
I also want to take into account that member_function might have more than one parameter. That is, the first parameter will always be something known but there might be other defaulted parameters.
The only thing I can think of is to make a proxy structure, something like this:
template<typename T, T> struct proxy;
template<typename T, typename R, typename... Args, R (T::*member_function)(Args...)>
struct proxy<R (T::*)(Args...), member_function> {
R operator()(T &obj, Args.. args) {
return (obj.*member_function)(std::forward<Args>(args)...);
} }
Which would then allow me to do (for example) this:
Foo<std::string, proxy<void(std::string::*)(), &std::string::clear> >
when Foo is implemented like this:
template<typename T, typename member_function>
class Foo {
public:
void someFunction() {
T t;
member_function()(t);
} };
That implementation works for me.
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May 30, 2013
I've was trying out a function template to automatically get the type of a lambda, but it seems that it won't compile
I've tried two different ways:
1.
template<class HASHER>
auto make_unordered_map(size_t bucketCount, HASHER const && hf)
-> unordered_map<string const, HASHER>&& {
return unordered_map<string const, int, HASHER>(bucketCount, hf);
} auto x = make_unordered_map(1, [](string const& key)->size_t { return key[0]; });
2.
template<class HASHER>
auto make_unordered_map(size_t bucketCount, HASHER const && hf2)
-> unordered_map<string const, int, decltype(hf2)> {
return unordered_map<string const, int, decltype(hf2)>(bucketCount, hf2);
} auto x = make_unordered_map(1, [](string const& key)->size_t { return key[0]; });
The test code are located here:
1. [URL] ....
2. [URL] ....
They are both based on the code that is stated to work in those examples. I.e.:
auto hf = [](string const& key)->size_t { return key[0]; };
unordered_map<string const, int, decltype(hf)> m (1, hf);
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Sep 19, 2014
Basically I'm trying to pass an object as a reference to the template function, rather than a copy as it's seeing. I'm needing to do this without editing Obj::Call to accommodate a reference as its first parameter, as it'd break other calls.
You'll notice in the following code the object will be destroyed upon passing, while the object defined is still in-scope due to the infinite end loop.
#include <iostream>
#include <string>
using namespace std;
class Obj {
public:
string name;
Obj(string name): name(name) {cout << "create " << this << endl;}
[code]....
In the past I tried ref(), which appeared to stop this happening, however it created a blank copy of the object instead.
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Nov 2, 2014
how I want the code to look. Only problem is it doesn't work (Line 11). I have some experience with templates but I'm not a pro.
Basically I want the "Channels<3>" to be a type that I can use to specify a Cable with similar to vector<float/int> it would be Cable<Channels<2 or 3>>.
What have I messed up with the syntax?
#include <iostream>
#include <vector>
using namespace std;
[Code].....
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Dec 11, 2014
I have been trying to get a hang on templates. I have the two following functions that that could be consolidated in a single template function:
void Attractor::updateFamilies(FamiliesController *_tmp, int _counter){
center.x = ofGetWidth()/2;
center.y = ofGetHeight()/3;
attractorCounter = _counter;
if(attractorCounter == 1){
[Code] .....
NotesController and FamiliesController have the same parent. The thing that I'm trying to grasp with templates is that is could something like:
template<class TYPE>
void Attractor::updateData(TYPE* *_tmp, int _counter){
center.x = ofGetWidth()/2;
center.y = ofGetHeight()/3;
attractorCounter = _counter;
[Code] ....
And then have another template function declaration for all the attractor functions where I pass the same template value as in the first one.
As you can see, I'm calling another functions inside called attractors(_tmp). I know that one way around it could be to get rid of that function and just do all the logic inside of each if statement. Is there any way to pass the same template function parameter within a template function parameter?
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Jan 16, 2014
Unless I'm missing something, it's now possible(ish)? A little concept is below, very rough around the edges. Still though, if this is valid by standard C++, why can't we have built-in support for float / double template parameters?
#include <iostream>
#include <tuple>
template<int Numerator, int Denominator>
struct Float {
constexpr static float value()
[Code] ....
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Sep 18, 2014
I have the defined a class Seconds with a template<int,int> constructor. Unfortunately, I can't seem to get an instance. Here's the class :
class Seconds {
public :
template <int num, int den> Seconds(long int);
}
And when I try to instantiate it :
Seconds *millis = new Seconds<1,1000>(30); // template argument deduction/substitution failed:
// couldn't deduce template parameter ‘num’
Is it not the right way to call the constructor?
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Mar 8, 2013
I have a triple hierarchy class:
template<class T> class Singleton;
class Base;
class Sub : public Base, public Singleton<Sub>;
I' using underlying auto pointers, that's why Singleton is a template class and Sub passes itself as a template parameter. I'm developing Singleton and Base and a public API allows anyone to add their own sub classes. I actually want a real triple hierarchy like this:
template<class T> class Singleton;
class Base : public Singleton<Base>;
class Sub : public Base;
So that external developers don't have to worry about templates and complexity. The problem with this is that my implementation in Singleton will now call the constructor of Base whenever I create an instance of Sub (since the template parameter is Base).I was wondering if this could be done by pre-processor macros:
template<class T> class Singleton;
class Base : public Singleton<__CLASS_NAME__>;
class Sub : public Base;
Where __CLASS_NAME__ is the class name that will be replaced by the pre-processor. Theoretically this should be possible, since the __PRETTY_ FUNCTION__ macro actually returns the class name. The problem is that one cannot do string-manipulation to remove the function name from __PRETTY_FUNCTION__.
how I can accomplish this so that the Sub class is not aware of inheriting from a Singleton<template> class?
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Nov 20, 2013
I've been trying to create a templated class that takes a template as a parameter. I'd like to specialise this class for certain partial specializations of the template parameter but can't seem to figure out how to do it nor find anything online, (although I may be searching for the wrong thing).
As an example, say I have a class A that takes a template class with two parameters as its parameter:
template< template<class X, class Y> class Z > class A {};
I'd like to have a general version of A, for a general version of Z, but a specialisation of A for a specialisation of Z, e.g. where X is int but Y is still any type.
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Apr 2, 2015
Is there an "easy" way to define a template with a template parameter that could resolve to "nothing".
Code:
template <typename T1, typename T2, typename T3>
struct one_to_three {
public:
T1 t1;
T2 t2;
T3 t3;
};
Now I want to be able to instantiate that in such a way that T3 and T2 (if T3 also) resolve to "nothing".
Don't point me at tuple<>, this has to be a single structure. No dummy's, nothing effectively means nothing.
so on Win32
Code:
one_to_three <int, int, int> x;
sizeof(x) == 12;
one_to_three <int, nothing, nothing> y; // whatever 'nothing' needs to be.
sizeof(y) == 4;
T1 will never be 'nothing'
T2 will only be 'nothing' if T3 also is 'nothing' (if it works with T3 not being nothing, that's fine, but it won't get used that way).
Portability is a non-issue, this only needs to work in VS (2010 and higher). The 'real' solution will need up to T10, I have a solution working with SFINAE, but it takes very very long to compile and it's getting very unwieldy if you would need to add T11.
I know it's not an ideal type approach, but it is what it is, this is a necessity due to linking with a legacy API which we don't have control over.
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Sep 19, 2014
Due to the nature of this requirement, I've made a very minimal example, which would adequately solve my issue, without resorting to use of pointers or copy constructors.
Basically I'm trying to pass an object as a reference to the template function, rather than a copy as it's seeing. I'm needing to do this without editing Obj::Call to accommodate a reference as its first parameter, as it'd break other calls.
You'll notice in the following code the object will be destroyed upon passing, while the object defined is still in-scope due to the infinite end loop.
#include <iostream>
#include <string>
using namespace std;
class Obj {
public:
string name;
[Code] ....
In the past I tried ref(), which appeared to stop this happening, however it created a blank copy of the object instead.
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Jun 13, 2013
void extf(int a) { }
template<typename P>
struct A {
// 1
template< void (*F)(P) >
static void call(P arg) {
[Code]...
Why it is not working? What would be a proper way to pass function pointer as a template parameter?
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Jul 27, 2012
Is this really the preferred way to get the return type, for use in a derived class, of a function defined in the template parameter?
template<class PARAMETER> class C {
protected:
typedef typeof (reinterpret_cast<PARAMETER*>(0))->function() returntype;
};
This works just fine for me, but seems inelegant.
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Oct 14, 2014
I'm looking for a way to generate a program-wide unique value to use as a template parameter. Generating a unique value within a translation unit is pretty easy with __LINE__, but that doesn't ensure uniqueness across translation units. I thought maybe I could use __FILE__, but that can't be used as a template parameter.
I stumbled across this page: [uRL] ....
which is exactly what I want, except that the anonymous namespace trick doesn't work on all the compilers I've tried it on. (This may be due to C++11 changing anonymous namespaces to internal linkage rather than external as they did before....that page is five years old.)
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Aug 23, 2013
Code:
#include <stdio.h>
void ASCII_to_EBCDIC( size_t, unsigned char *);
void EBCDIC_to_ASCII( size_t, unsigned char *);
void to_ASCII(unsigned char *);
void to_EBCDIC(unsigned char *);
/* conversion tables */
static unsigned char
[Code] ....
The above snippet is for a buffer/string, where as i want to pass file name as a parameter and want function to process the file line by line?
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May 1, 2013
#include <cstdlib>
#include <iostream>
struct tax_node {
char form; // tax form letter
int version; // tax form number
[Code] ....
I cannot seem to get why function print_contents will not work. The couts at the end of the program is just to test that it printed correctly. But, if I need it to print the contents such when print_contents(ptr2) is called. I think it should be tax_ptr in the parameter list but I am not quite sure.
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Jan 7, 2015
gcc v.8.3 -std=gnu++11
[URL]
I'm trying to pass a function as a parameter and failing. It seems simple, until I get the error messages.
Here is the code:
class MinimalSolver {
typedef double (*func)(double sum, double char);
void driver();
[Code]....
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Nov 16, 2014
I want to create events and then, functions which are subscribed to the event can access information about the event. For example, in Class 2 below, I want it to be able to access things such as touch.position, etc. of class 1.
Class 1:
public delegate void TouchEventHandler (EventArgs e);
public event TouchEventHandler TouchBegan;
public Vector2 touchPosition;
void Update () {
if (Input.touchCount > 0) {
[Code] ...
Class 2:
void OnTouchBegan (EventArgs e) {
Debug.Log ("Ran");
}
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Oct 6, 2014
How come when we pass an array as a parameter to a function, the entire array is copied and is available to function?
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May 2, 2013
Write a program that inputs 10 integers from the console into an array, and removes the duplicate array elements and prints the array. By removing, I mean that you should make it appear as if the elements hadn't been there. You may assume that all the integers are between 0 and 100, Write at least 1 function in addition to the main function, and pass an array into that function as a parameter. e.g.
Please enter your 10 numbers: 1 2 3 4 5 6 7 8 9 10 The array contains: 1 2 3 4 5 6 7 8 9 10
Please enter your 10 numbers: 1 1 3 3 3 6 7 8 9 9 The array contains: 1 3 6 7 8 9
Please enter your 10 numbers: 1 1 1 1 1 1 1 1 1 1 The array contains: 1
The bolded area is where I'm having trouble. How I can go about doing this, passing an array into the function as a parameter?
Here is my code:
#include "stdafx.h"
#include <iostream>
using namespace std;
int main () {
const int MAX = 10;
int a[MAX] = {0};
int i;
[Code]...
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Sep 25, 2012
I created the following code to pass the the variable 'inputVoltage' by reference to the function 'input'. It certainly works when I run the program, but I dont think it is a standard way of doing it, i.e. the use of '*' and '&' is not according to convention ? Or perhaps the way did it is acceptable ?
int input (double *inputVoltage);
int main ( {
double inputVoltage;
input(&inputVoltage);
[Code]....
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Jul 29, 2013
I have a class and I would like to be able to pass an extra parameter to the function that is executed.
BigInt operator / (BigInt N,BigInt D) {
...
}
is what I have now. but I would like to do something like this. so the default value for a is 10. and if the user does something like N/D (12) Then the value of a is 12.
BigInt operator / (BigInt N,BigInt D, int a=10) {
...
}
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Dec 17, 2014
Here is my issue: I am making a simple audioplayer in Xamarin.android but i want every time i change the track to make a crossfade effect. So im using 2 mediaplayers at the same time for the fade. The problem is that im defining one time the players and i pass the player as a parameter like this:
public MediaPlayer player = null;
public MediaPlayer player2 = null;
....
If i have to fadeout the player and start the next one im doing it like this:
if (player != null){
if (player.IsPlaying) {
cts = new CancellationTokenSource();
token = cts.Token;
FadeOut (player, 2000 ,token);
[Code] .....
So my problmem is that player and player2 remain always null. Why? i guess c# creates a copy of player and player2 and use this one. How i can pass a mediaplayer as parameter and always use only player and player2?
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Jan 24, 2015
Does putting a ref when passing a struct to a parameter work? Say i have this code
struct struct1 {
public int i;
public void show() {
Console.WriteLine(i);
[Code] ....
The output doesn't change. The result is
0
100
500
0
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Jun 11, 2013
I have a class matrixType that has some overloaded operators (+, -, *, and <<). With a view to having clearly-delineated, perfectly-formatted, four-sided matrices, as shown below:
A = 1 2 3
4 5 6
7 8 9
or
A + B = 1 2 3
4 5 6
7 8 9
and NOT this jagged ones shown below:
A = 1 2 3
4 5 6
7 8 9
or
A + B = 1 2 3
4 5 6
7 8 9
,
I want a scheme in which the string literals (A, A+B, etc.) could be passed as parameters to the overloaded stream insertion (<<) operator function so that I could use the string’s length to determine how much offset from the display screen’s left to apply to each matrix’s row (by using the setw() function). However, I do know that the << operator is a binary operator, meaning the function cannot take more than two parameters: that is what compounds my problem!
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