C++ :: Enable If With Variadic Template Arguments
Sep 17, 2014
template <typename FIRST, typename... REST, typename std::enable_if<std::is_convertible<FIRST, Base*>::value>::type* = nullptr>
void foo (FIRST first, REST... rest) {}
that successfully allows me to enable the function foo() only if FIRST is convertible to Base*, but I also only want foo() enabled if each type in REST... meets the same condition. What is the syntax for that? If no such syntax exists, how to achieve that effect?
View 7 Replies
ADVERTISEMENT
Feb 1, 2013
I'm trying to expand a template + argument parameter list inside a lambda function like this:
template <typename Class, typename ...Args>
static void create(Args ...args) {
// Perform pre-thread creation work.
std::thread([=]()
[Code] ....
But this does not work:
The compiler error is "error: parameter packs not expanded with ‘...’:|"
However, when I do the following:
template <typename Class, typename ...Args>
static void create(Args ...args) {
// Pre-thread work.
auto tthr = [](Args ...ar) -> void {
[Code] ....
It works just fine. That shows that lambda threads are able to take variadic arguments...
So here is my question; what is the correct capture clause for capturing the variadic object correctly?
View 3 Replies
View Related
Jul 31, 2014
I had to learn how to use variadic templates recently, and had trouble finding simple examples that just showed the basic syntax.
So I decided to write one myself. Admittedly, it's a bit on the long side, but that is mostly because it includes five specializations.
insert Code:
// Variadic.C
// Compile command: g++ Variadic.C -std=c++0x
// I used GCC version 4.6.3 on Ubuntu.
// This file contains a basic variadic template with five specializations.
// It is intended for non-software engineers who are looking for a simple
// example of variadic template syntax.
[Code] ....
View 3 Replies
View Related
Nov 1, 2013
I wonder if it is possible to remove the last argument in an argument pack? Below is an example on what I want to accomplish:
template<template<int...> class A,int... Ints>
A<remove_last_int<Ints...>::list> func(const A<Ints...> & a0)
{
A<remove_last_int<Ints...>::list> a;
...
//Here a set the members of a based on a0.
...
return a;
}
For example, I want the return a A<1,2> value from (const A<1,2,3> & a0)
View 8 Replies
View Related
Dec 14, 2014
I'm having some problems in understanding how the code below works and why it produces the output it produces.. What I'd expect is that both functions, namely `add_1' and `add_2', would print the same output; but I've been proven wrong :/ So why does the second one get different memory addresses for the same variable?
Code should be self-explaining:
Code: template<typename... Types>
void add_1(Types&&... values)
{
// by the way: why do i have to use `const int' instead of `int'?
std::vector<std::reference_wrapper<const int>> vector{
std::forward<Types>(values)...};
std::cout << "add_1:" << std::endl;
for (const auto& value:vector) {
std::cout << &value.get() << std::endl;
[code].....
View 4 Replies
View Related
Oct 30, 2013
When compiling the code
#include "tensor.h"
int main() {
Tensor<2,-2> m = {{1,2},{1,3}};
Tensor<2> v = {1,5};
std::cout<<m*v<<"
[Code] ....
Why do I get an ambiguity and why is not the wanted operator*-overload (the last one in the tensor.h file) not even mentioned as one of the candidates? Is it clear what I want to do? And if so, what can I do to make the call unambiguous?
View 3 Replies
View Related
Oct 24, 2013
I have been experimenting with variadic templates with the aim of caching a call to a class method by storing away the object pointer, method pointer and parameters. I've actually had some reasonable success but have now hit a stumbling block. I now wish to wrap my parameters in a simple template class when I cache them. My success is as follows:
Using variadic template functions to store these pointers and paremeters;
I'm able to pass a method pointer and unwrapped parametersI'm able to pass wrapped parameters on their own.I'm NOT able to pass a method pointer and wrapped parameters I set up a little prototype project to demonstrate the issue and added comments above the function calls to indicate the compilation results. Here is the code:
Code:
#include "stdafx.h"
//////////////////////////////////////////////////
// Basic class with a simple method
//////////////////////////////////////////////////
class MyClass {
public:
char Method( int i, float f ) {
return 'A';
[code]....
But I'm convinced it should take three arguments, the method pointer and two wrapped parameters. Visual studio even suggested it should as shown below:
View 3 Replies
View Related
Mar 14, 2014
I have a function:
template<class Iterator, class T>
void a(Iterator, Iterator, const T&);
and I want to be able to simplify calls to 'a' with calls like
a(someIteratableContainer);
instead of having to call:
a(someIteratableContainer.begin(), someIteratableContainer.end(), valueOfTheContainersElementType);
I also want to be able to generalize the function to handle any of the standard iteratable contains: array, vector, deque, whatever.
I was under the impression I could write:
template<template<class T> class U> a(U<T>& container) {
a(container.begin(), container.end(), g(T()));
}
where 'g()' returns an object of the element type. However, the compiler is claiming, no matter how I write a call to the overload, the original template is selected and/or the overload is invalid, depending on the various ways I attempt to write said overload.
View 7 Replies
View Related
May 26, 2013
I'm trying to create a callback wrapper for pointers to member functions of any class by using templates, std::function and std::bind. This will be used to send incoming sf::Event's to classes who register callbacks with an event manager. I based my code off of the example on this page: URL.....Here's what I have:
class EventCallback
{
std::function<bool(const sf::Event&)> func;
public:
template<typename T>
EventCallback(T* object, bool(T::*func)(const sf::Event&)) { func = std::bind(func, object, std::placeholders::_1); }
bool run(const sf::Event& evt) { return func(evt); }
};
[code]....
View 4 Replies
View Related
Nov 7, 2013
Today I faced a problem where I had circular dependency in my template arguments. I was trying to make a class hierarchy similar to:
template<class BType>
class A_base {
public:
BType* getB();
};
[Code] .....
Basically I had objects that were of type A<B<A<B<...
Basically I have a tree like structure of heterogeneous types that must facilitate two-way interactions where A's can call B's and B's can call A's. This structure is useful in many contexts the difference is the methods A and B provide are different in each of these contexts. Instead of adding the getA and getB and all the other connectivity methods in every version of A and every version of B, I wanted to create a base class that managed this automatically.
Another piece of advice was break up your code so there is a forward-only and backwards-only dependent types. This is not a complete solution because the two cannot know about the other and this does not really facilitate arbitrary two-way communication (where A calls B then B calls A back). It also makes the code more complicated in that I have two sets of objects and interfaces.
So the solution was to make the template arguments specific to the things I wanted to be flexible. The connectivity interface of A_base and B_base should be constant. Hence that cannot be in the template parameter. It was merely the traits that I wanted to make flexible so... I came up with this solution:
#include <iostream>
template<class aTraitType,class bTraitType>
class A;
template<class aTraitType,class bTraitType>
class B;
[Code] ....
Now this compiles and works great. The problem is that aObj and bObj cannot call their opposite within a trait method because print() does not know anything about the connectivity. So the solution there was to make traits an abstract base class. Then magically everything works!
#include <iostream>
template<class aTraitType,class bTraitType>
class A_base;
template<class aTraitType,class bTraitType>
class B_base;
[Code] .....
So this outputs the following. Clearly there is two-way communication!
Class A is not connected to B
Class B is not connected to A
Class A at 0x7fff25d1aa10 reporting for duty
Class B at 0x7fff25d1aa00 reporting for duty
Class B at 0x7fff25d1aa00 reporting for duty
Class A at 0x7fff25d1aa10 reporting for duty
Class A at 0x7fff25d1aa10 reporting for duty
Class B at 0x7fff25d1aa00 reporting for duty
View 6 Replies
View Related
Feb 4, 2015
For example I want to get the type of std::vector<>::size_type without specifying a template argument.
Here's what I want to do:
std::vector<>::size_type x{5};
Is there any way this can be done?
View 4 Replies
View Related
May 22, 2013
working on this code I have encountered a few issues. My program lists the occurrence of each letter but i'm unsure of how to enable numeric variables. how to improve the overall quality of the code.
Code:
#include <stdio.h>
#include <string.h>
int main()
{
char string[100], ch;
int c = 0, count[26] = {0};
printf("Enter a string
[Code]...
View 4 Replies
View Related
Jun 22, 2014
i have tried to search on the Internet but haven't found a suitable result that links in with what I want to do, that is to creation a WPF application and when you press a enable button it will enable the LAN and when I click the disable button it will disable the LAN until i click the enable button again. Now I know how to creation a WPF and the buttons, but how would I do the disabling LAN and enabling LAN?
By LAN i mean just the networking adapters, so that the PC cannot connect to a router and then the Internet.
View 14 Replies
View Related
Nov 19, 2013
I have ploughed through the exchanges in all the forums, yet I still cannot make it work. The following is a tiny Code::Blocks program, written to isolate the problem:
// resource.h
#define IDR_MYMENU101
#define ID_FILE 102
#define ID_DISABLE103
#define ID_ENABLE 104
#define ID_MESSAGE105
#define ID_EXIT106
[code].....
case ID_ENABLE is similar to case ID_DISABLE above, and neither works. case ID_MESSAGE and case ID_EXIT both works as expected.I have tinkered with EnableMenuItem(), but without success.
View 8 Replies
View Related
Dec 5, 2012
I have a version of an app in 64bit but the problem is under program files on windows 8 64bit it doesn't have access to the programs files folder... usually virtualization will redirect it to the virtual store but with 64bit exe's this doesn't happen automatically. How do I enable it in code or in the manifest file?
From MSDN:
Virtualization is only enabled for:
32 bit interactive processes
Administrator writeable file/folder and registry keys
Virtualization is disabled for:
64 bit processes
Non-interactive processes
Processes that impersonate
Kernel mode callers
Executables that have a requestedExecutionLevel
View 1 Replies
View Related
Dec 27, 2013
Arrays are by far the most fun to work with and I really need see if its possible to enable an array to count apples, pears, peaches, from a group of three each.
If any of the fruit groups were chosen how might an algorithm be put together to count how many were chosen from each group. But lets take into consideration that these fruit groups are not supposed to be randomly generated, instead they belong as an array with certain amount of chosen occurrences. To show how many fruits were chosen from each group example, 1 from group 1, 2 from group 2, 3 from group 3 the out put is the important part because it shows the group and the occurrence of that group. the algorithm should be able to display the group with an occurrence as well
View 1 Replies
View Related
Nov 26, 2012
The example enable a client to iterate the internal std::vector using being() and end().
Code:
class foo {
public:
typedef std::vector<std::string>const_iterator iter;
iter begin () const;
iter end () const;
[Code] .....
In the future I see the need for this class to be able to control sequence (sorting) and also show a subset of the complete list based on a search parameter.
Using std::sort appear to solve the ability to sort the collection.
How can I return an iterator to the client which only iterates a sub-set of all items in the std::vector?
An example would be, I add this method to the class;
Code:
void find(const std::string& st);
So if the client performs (below) only items in std::vector that contains the character "a" should be possible to iterate.
Code:
foo f;
f.search("a");
One option would be to operate with two collection inside the foo class. One more static containing all items and the other containing the sorted and filtered items. This would lead to some copying but should work. Far from perfect.
View 2 Replies
View Related
Sep 12, 2013
I'm doing right now is creating a function that callocs (I prefer this to malloc) and returns a string, and it will work similar to printf, I'm calling the function alloCpy(),I have several values that I need in a malloced string, so I call Code: myAllocedString = alloCpy("Value 1 is %s, value 2 is %s, and value 3 is %d", str1, str2, num); To do this I'm using the Variadic Macro, the reason I'm not just using a Variadic Function such as this: Code: char* alloCpy(char *format, ...) {} is because I need to append NULL to the end for the sake of looping through arguments, and I'm understanding it thusfar, but I have a few issues, first of all, I tried defining the Macro in a header file, but when I try to call it I get the error "Undefined reference to alloCpy". Also, to loop through arguments to get string lengths I'm using va_arg(args, char*) which requires all the arguments to be of type char*. Here is my code:
myheader.h:
Code:
#define alloCpy(format, ...) _alloCpy(format, ##__VA_ARGS__, NULL);
char* _alloCpy(char *format, ...); mycfile.c: Code: char* _alloCpy(char *format, ...) {
va_list args;
va_start(args, format);
int args_len = 0;
}
[code]....
So, how can I do this to, first of all, make my macro function accessible from other files importing myheader.h, and second, how can I make it accept any type of argument like printf, so that my example above would work?
View 3 Replies
View Related
Jan 30, 2014
I have a variadic base class with a pure-virtual function per type:
Code:
template <typename ... Types>
class Base;
template <typename T, typename ... Types>
class Base<T,Types...>: public Base<Types...>
{
public:
using Base<Types...>::doSomething;
[Code] ......
Now, I'd like to inherit from it using another variadic class, which provides implementations of doSomething(), but I run into trouble --- where do I indicate it derives from Base?
Code:
template <typename ... Types>
class Derived;
template <typename T, typename ... Types>
class Derived<T,Types...>: public Derived<Types...>
[code]....
I see two possible approaches:
First, virtual inheritance *might* get me what I want, but I don't know how bad a performance hit that might be.
Second, I could do some magic where the full set of types is captured in a tuple at the lowest level and continually passed up, then re-expanded in the base case to indicate Base inheritance. However, I'm not sure if that can be done in an unambigious manner; I can't have two variadic packs at once (Types... and the tuple contents), and I'm not sure if there's a way to use enable_if to check if an arbitrary template type is any kind of tuple.
View 2 Replies
View Related
Apr 29, 2015
Using C . I have been tasked (for a University assignment) to create a program that will enable a user to upload an image and convert that image into ASCII text using SDL on a Linux system. I've managed to open a window, display an image (non-selected) and convert it into grayscale using
case SDLK_g:
for (int y = 0; y < image->h; y++) {
for (int x = 0; x < image->w; x++) {
Uint32 pixel = pixels[y * image->w + x];
Uint8 r = pixel >> 16 & 0xFF;
Uint8 g = pixel >> 8 & 0xFF;
Uint8 b = pixel & 0xFF;
Uint8 v = (0.212671*r)+(0.715160*g)+(0.072169*B)/>;
pixel = (0xFF << 24) | (v << 16) | (v << 8) | v;
pixels[y * image->w + x] = pixel;
I am absolutely clueless as to how I can get the user to upload an image to the program and then begin the image -> ASCII conversion. I've found seem to be written in C++ or C# to make the conversion I should be using the GetPixelColour command?
View 14 Replies
View Related
Jan 28, 2014
Let's say I have a variadic type A and a non-variadic type B:
Code:
template <typename ... Args>
class A
{};
template <typename T>
class B
{};
Now, I'd like to do the following (pseudocode since I don't know how to do this yet):
Code:
template <typename ... Args>
class C: public A<B<Args1>,B<Args2>,B<Args3>....>
{};
Essentially, I'd like to take the parameter pack used for C and compose B around each element individually, then pass the result to A.
how I can do this?
View 2 Replies
View Related
Jan 2, 2014
I'm trying to learn how to use variadic templates, and I decided a great example would be serializing a series of types into a stringstream:
Code:
// Send a fully constructed message.
virtual void send(ostringstream &msg) = 0;
// Construct a message from the arguments and send it.
// This is the usual entry point.
template <typename ...Args>
void send(Args ...args {
[Code] ....
This works fine, so far as I can tell. However, I decided to see if I could specialize the way certain types are serialized. I tried using a Google Protocol Buffer object as an example, and added this:
Code:
// Handle a protocol buffer type while constructing a message.
template <typename ...Args>
void send(ostringstream &msg,
const google::protobuf::MessageLite &protobuf, Args ...args) {
std::string msg_str = protobuf.SerializeAsString();
msg << msg_str;
send(msg,args...);
}
I would expect this overload to be preferred over the generic T overload when a protobuf object (which always inherits from MessageLite) is passed into send() anywhere in the list. However, this is not happening. I am getting an error message to the effect that << doesn't know how to deal with my concrete type, pointing at the T overload.
View 4 Replies
View Related
May 13, 2013
Perhaps this can be a very popular error,
Code:
#include <stdio.h>
#include <tchar.h>
#include <iostream>
#include <string>
#include <fstream>// enable writing to and reading from files
#include <cstdlib>
#include <time.h>
using namespace std;
class Person {
[Code] .....
Error list:
Code:
Error 1 error LNK2019: unresolved external symbol "void __cdecl saveperson(void)" (?saveperson@@YAXXZ) referenced in function _main H:Cry_DevProgrammingC++Using_class_ in_ c++Using_class_ in_ c++Using_class_ in_ c++.obj
Error 2 error LNK2019: unresolved external symbol "void __cdecl displayperson(void)" (?displayperson@@YAXXZ) referenced in function _main H:Cry_DevProgrammingC++Using_class_ in_ c++Using_class_ in_ c++Using_class_ in_ c++.obj
Error 3 error LNK2019: unresolved external symbol "void __cdecl editperson(void)" (?editperson@@YAXXZ) referenced in function _main H:Cry_DevProgrammingC++Using_class_ in_ c++Using_class_ in_ c++Using_class_ in_ c++.obj
Error 4 error LNK1120: 3 unresolved externals H:Cry_DevProgrammingC++Using_class_ in_ c++DebugUsing_class_ in_ c++.exe 1
What I can do to fit this problem?
View 14 Replies
View Related
Nov 17, 2014
I noticed that when using variadic functions, if I pass the va_arg() as parameter to a function, the parameters get passed in reverse. Is that expected?
For example, the following code outputs
Code:
1 2
2 1
Code:
#include <iostream>
#include <stdarg.h>
void foo_func(int v1, int v2)
{
std::cout << v1 << " " << v2 << std::endl;
[Code] .....
View 3 Replies
View Related
Feb 25, 2013
I'm looking for a way to enter an unlimited amount of types in the <> part of a template function, I found Variadic templates but I'm not sure if it can do it, all the examples I've found are similar to the C argument list and don't use the <> part of the template at all.
I tried this, but the overload is ambiguous?
#include <iostream>
#include <typeinfo>
template<typename T>
void stuff() {
std::cout << typeid(T).name() << "
[Code] ....
View 2 Replies
View Related
Dec 5, 2013
I'm trying to implement a simple template array class, but when i came into the operator< i actually have to use a template :
my code is something like :
template<typename _Type, std::size_t _Size>
class array {
public :
[Code] ......
but i am having an error of shadows template param 'class _Type' is it w/ the name conflict between the array template parameter and the function template parameter ?
View 6 Replies
View Related
