C++ :: Variadic Template Basic Syntax
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] ....
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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)
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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?
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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].....
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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?
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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?
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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:
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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?
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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.
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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?
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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.
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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] .....
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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] ....
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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 ?
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Nov 6, 2013
Error1error C2955: 'DoubleLinkedListInterface' : use of class template requires template argument listdoublelinkedlist.h10
Error2error C2244: 'DoubleLinkedList<T>::DoubleLinkedList' : unable to match function definition to an existing declaration doublelinkedlist.cpp7
Error3 .cpperror C2244: 'DoubleLinkedList<T>::~DoubleLinkedList' : unable to match function definition to an existing declaration 12
.h
#pragma once
#include "DoubleLinkedListInterface.h"
#include "Node.h"
#include <iostream>
[Code]....
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May 27, 2013
I have a generic template class with another template in one of its types. Now I want to specialize one of its methods for a particular (template) class, which leads to a compile error, however.
Here is the example:
#include <stdio.h>
template<typename Type>
class Obj1 {
public:
void ID() { printf("Object 1, size = %zu
[Code] .....
GCC ends with:
:35:27: error: type/value mismatch at argument 2 in template parameter list for ‘template<class Type, template<class> class O> class Foo’
:35:27: error: expected a class template, got ‘Obj2<Type>’
What is wrong with the specialization? Can it even be achieved and how (if so)?
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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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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.
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Feb 3, 2013
I am using code::blocks for c programming and when i take debugger in below code it show ..
a syntax error in expression near "if"..
I am just checking debugger ...
Code:
#include <stdio.h>
#include <string.h>
int main()
{
char s1[100];
[Code] ....
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Jun 15, 2013
Q. In context of C language syntax checking, which of the following can be modeled using Finite Automata?
(A) Detecting proper termination of an instruction.
(B) Detecting balance of parentheses.
(C) Detecting initialization of a variable.
(D) None of the above.
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Feb 28, 2014
I am just starting out with loops and I have run into an syntax error and for the life of me I cant find out the issue. Before it has been an issue of me forgetting to close my brackets but I am pretty sure I did that this time. I am trying to have the program keep looping the main menu until the user inputs a correct number.
float total_Bal = 0.00;
char user_Choice;
do
{
[Code]....
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Feb 5, 2014
I'm familiar with function pointers as the method to pass a function as an argument to another function. However, I recently encountered some other syntax. I have seen in multiple times and in books,so I know it is probably not a syntax error. But I am not totally sure.
#include <iostream>
void Function1() { std::cout << "Function1"; }
void Function2(void aFunction()) { aFunction(); }
// make a param that describes the function the will be given
// as a param (return type, identifier,
[Code].....
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Apr 20, 2012
I keep getting a "Declaration syntax error" at line ""int main()". Is there something wrong with my int main()? And how do I go about it? Here is the program:
#include<stdlib.h>
#include<iostream.h>
#include<stdio.h>
#include<math.h>
#include<conio.h>
float rung4(float x, float y, float h)
int main() {
float eps=0.00001;
[Code] .....
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Feb 19, 2012
I got syntax error in if statement ,, i checked the line i put { after the condition don't know where the mistake are
1>c:usershani est11 est11code.cpp(20) : error C2143: syntax error : missing ';' before 'if'
PHP Code:
# include <iostream>
using namespace std;
int seqsearch (int list[],int length,int key);
void main () {
int marks [30];
[Code] ....
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Mar 7, 2014
I need to run some basic statistics on about ~150 tables that are currently in dBase format. Rather than use excel to do them all individually, i was told to use s+ and write a code to loop through them, but I have never written any code and my experience with spotfire s+ is limited (the stat software i have available) ....
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Nov 17, 2013
[URL]
#include <iostream>
struct Outer {
template<typename T>
void go() {
std::cout << Test<T>::f() << std::endl;
[Code] .....
I have tried several variants on this code to no avail. Outer is in a header, along with the extern template statements, and the specializations after main are in their own cpp file. Main is in a different cpp file.
What do have to do to make this work? I cannot bring the definitions of f() into the header, and they will be different for different template parameters. Ideally, I want Test to remain a private member of Outer, though this can change if it's the only option.
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