Visual C++ :: Template Parameter Resolving To Nothing?
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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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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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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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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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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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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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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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 5, 2013
i have this Stealth Injector source from internet so this program is for injecting .dll to an .exe this program was made by someone to used for cheating in online game but i need to use this program in my private server game online to tell the game client .exe the server IP, that is stored in a dll file.. the problem is i don't want the player to directly execute this program, but they need to run the game patcher first to do the patch.. so i need to put some secret parameter argument that will block player from direct execute..
i know nothing about c++ i only know that you need to use main(int argc, char *argv[]) i've try to put something like this
Code:
int main(int argc, char* argv[]){
stringstream sparam;
string param;
[Code].....
the code works fine but the rest of code won't executed..
i've attached the cpp and header files for you guys to check source.rar
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Feb 12, 2014
I need to pass a variable to a dialog box.
Code:
Doc* pDoc;
Dialog dlg;
int input = dlg.DoModal();
When I call dlg.DoModal() I need to somehow pass the pDoc into the dialog box. Everything I need the variable for is taking place inside the oninitdialog function. Is there anyway to pass the variable to that function?
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Sep 11, 2014
So I'm currently using Visual Studio 2013, and I was left for homework as a beginner programmer to calculate two summations. First the summation of k to n. k=1 and the equation being k^3. The parameter would be n and I need to printf the results for n=5, 33, 100, 442, 3456.
I know that some of the numbers are big values so float or long should be used but how to define these as of yet.
Another is to find the summation
(n)
(k) <<-- one big paranthesis
for values k=8 and n=10, 21, 35, 46, 68
How to start excluding <stdio.h> and int main ... Also, do I have to repeat a new function for every number for n?!
This is what I have so far for something I did differently
// Factoriales Sumatorias
#include <stdio.h>
int suma1 {
int n = 5; // Defino e Inicializo la variable
long sumatoria = 1; // definicion e inicializacion del resultado
while (n <= 5)
[Code] .....
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Jul 24, 2014
[URL]
class CMyclass
{
public:
CMyClass(BOOL bUseWhichMemManager);
void* operator new(size_t);
void operator delete(void*);
};
I create two memory manager called CMemManager1 and CMemMangaer2, using different algorithms to allocate buffer. Now I want to control which memory manager to be used, when calling new.
I try to add a parameter bUseWhichMemManager to the constructor, but in overrided new function, there are no way to access the parameter. Is there a way to pass more parameters to new operator, such as:
void* operator new(size_t size, BOOL bUseWhichManager);
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Jan 13, 2014
I have a Windows service that may change the timeout of the logon screensaver in Windows (as described here.) To do that I change the following registry key to the timeout in seconds:
Code:
HKEY_USERS.DEFAULTControl PanelDesktopScreenSaveTimeOut
The issue is that how do I make OS "read" or refresh the actual screensaver timeout after a change in the registry key above?
My practice shows that it is refreshed (for sure) only when I reboot the system, but in my case I need it to be applied without the reboot.
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Nov 25, 2012
i need to write a function with an array parameter and an int parameter.
that array has to be filled with first 10 prime numbers that are exact or higher than the int parameter...and then i need an average value of those 10 prime numbers...
The problem is im not really sure how i should do the part to fill the array with prime numbers that are higher than that int??
Code:
int avgprimearray (int higharray[], int somenumber){
}
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Sep 28, 2014
My errors are at the end of the program in two function calls within the definition of the InsertByValue function. g++ does not seem to recognize NumArray as a valid parameter.
#include <iostream>
#include <assert.h>
using namespace std;
const int CAPACITY = 20;
/* Displays the content of an int array, both the array and the size of array will be passed as parameters to the function
@param array: gives the array to be displayed
@param array_size: gives the number of elements in the array */
void DisplayArray (int array[], int array_size);
[Code] ....
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Apr 1, 2013
Can local variable be passed as the parameter for a new created thread procedure? Here is the example code:
Code:
void CDLG::some_function()
{
CString strFileName="abc.doc";
//local variable, can it be valid for being passed into the following new thread???
//Can strFileName still be accessed from within the stack of thread procedure?
::AfxBeginThread(ProcessContentThread,(LPVOID)&strFileName);
}
[Code]...
There is another method using variable on the heap,
Code:
void CDLG::some_function()
{
CString strFileName="abc.doc";
CString* pstrFN=new CString(strFileName);
::AfxBeginThread(ProcessContentThread,(LPVOID)pstrFN);
}
[Code]...
I test these code, both methods work as expected, but I doubt whether the first method is a good way. OR if only the second method is the correct way to pass a parameter to a thread.
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Sep 8, 2014
I am new to VC++ 2012. I have this code snippet.
Code:
auto it = query_map.find(U("callback"));
The problem is right under the dot there is a red line the error is
Error1error C2664: 'std::_Tree_iterator<_Mytree> std::_Tree<_Traits>::find(const http::uri::encoded_string &)' : cannot convert parameter 1 from 'const wchar_t [9]' to 'const http::uri::encoded_string &'d:maverickprojectsstrikeforcesrcserverserverserver.cpp26
What is the solution to this error?
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Dec 4, 2013
I am developing new project in Qt with existing MFC project . SO in MFC I have a function which uses SYSTEMTime and return CString.
example
CString getTimestampString( void )
{
SYSTEMTIME systemTime;
CString datestr;
[Code]....
PS -> I cant able to make any changes in lib_know as this library is being used by many other projects..
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Jul 2, 2013
Here's my function definition
bool validateNumber(string& text, int min = 0, int max = -1, bool useMin = true,
bool getValid = true)
The code takes the string text, and checks the make sure that the input is valid and safe to convert and use as a number. However, sometimes there is not min, and sometimes there is no max. The lack of min is done by using the parameter useMin, while the lack of max is done by max < min.
My predicament is the following call:
validateNumber(text, -2);
Now, max will be used, even though I don't want it. Ideally, I would want to do something like... int max = (min - 1), ... but that doesn't work. I also can't check to see if the parameter hasn't been changed (that I know of), because the following call would make it look like it hasn't
validateNumber(text, -2, -1);
So the question is, is there a way to do what I want, without having to add in a bool useMax parameter? Or is this my only option? I don't want to do that for simplicity, but if I have to, I have to.
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Mar 19, 2013
My question contains two projects that are under the same solution.
Project_1: I have created a dll where I use inside the project, OLE Db Template. In the project, I use this code.
class MyClass {
public:
LONG _value1;
SHORT_value2;
BEGIN_COLUMN_MAP(Accessor)
[Code] .....
Project_2: Is a project testing the Project_1.
- I want to call one of the exported functions.
- In Linker/Addional Dependencies, I have added the .lib
- In Linker/Additional Libray Directories, I have added the directory where is the lib.
- In C/C++/General/Additional Include Directories, I have added the directory on project 1. I did that because I want to #include a header in Project_1. But, when I do that, I get:
Error1error C3646: 'COLUMN_ENTRY' : unknown override specifier
Error2error C2059: syntax error : 'constant'
Error3error C3646: 'COLUMN_ENTRY' : unknown override specifier
I have done it several time with other projects, but the first time with OLE DB Template. atl90.dll is registered.
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