C++ :: Variadic Templates - Enter Unlimited Amount Of Types In Function
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'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.
Okay, so my assignment for my C class is to generate a program that will allow the user to enter data for a stock and calculate the amount of stocks that ended up being positive, negative, and neutral.I tried to do this using one stock, here is my code;
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:
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?
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;
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
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:
I'm trying to make a program that allows the user to input an arbitrary amount of numbers and finding the largest of all the inputs but I keep having problems with the output.
javascript:tx(' #include <iostream> using namespace std; //****************************************** //CLASS COMPARATOR //******************************************
class comparator { public: comparator();
[Code] .....
And regardless of what numbers I enter, I always get the output of 10. Also I got the EOF idea from my textbook so if there is a better way of doing this I'd like to hear it. I don't know any clear ways that looks nice to end the while loop when the user doesn't have any more numbers to enter.
I was just wondering how is this generally resolved. Let say you have this large function that runs in two modes. In the first mode it evaluates the data passed to a function as a map the the second mode it fills the map. example:
Code: template <typename Map, typename Int> void func(Map & map, Int i){ int z = 0; string zz;
[Code] ....
The point is i do not want to write a large function just to include different modes so i decided to set "i" to be a mode identifier. However when i want to compile my function given two modes i get an error since the modes are not recognized (obviously). if i pass map as
Code: map<int,int> and mode 1 i get an error here : Code: map[z] = z; besause map Code: map[z] expects z to be an int not string and the other way around (though in practice this cannot happen since i set the modes). So am i restricted to writing my function for both modes separately (polimorf.) or there is a way to make my example work.
i'm new to C++ i came across this bubblesort program earlier relating to class templates i was wondering how to make this into simple function templates
#include<iostream> #include<iomanip.h> #include<conio.h> #include<stdio.h> using namespace std; template <class t> class bubble
template <class T> void Arreglo<T> :: Registro (ifstream& Entrada) { Entrada >> Cantidad; Dealer = new T [Cantidad]; for (int i = 0; i < Cantidad; i++) {
[Code] .....
It says the following error when I comile it:
error: expected type-specifier before 'Detail' (*(Dealer + i)).Modelo = new Detail[(*(Dealer + i)).AmountModels]; error: expected ';' before 'Detail'
Why will i use explicit instantiation of a template, for a type? If I do not use explicit instantiation, the template is used to create the necessary function then what is the use of explicit instantiation?
In the above eg. explicit instantiation is done for char type. What is the use of this?? If the compiler can make use of the template to make a fn for char type.
I've written the following code and keep getting the errors:
Error1error C2244: 'Supermarket<temp>::operator =' : unable to match function definition to an existing declaration Error2error C2244: 'Supermarket<temp>::setName' : unable to match function definition to an existing declaration Error3error C2244: 'Supermarket<temp>::setArea' : unable to match function definition to an existing declaration
#ifndef SUPERMARKET_H #define SUPERMARKET_H
#include<string> #include<iostream> using namespace std;
[Code] .....
I moved the files to the .h file, and now I'm getting
So, I'm supposed to do : Create a function with unlimited number of arguments, which forms a dynamic string based on the following form (%d, %s, %f, %lf, %c), with the following prototype:
char*create(char*form, ...);
The function is supposed to have the following output:
create("Peter is %d years old and is in %s-%c class.",7,"second",'A'); -> Peter is 7 years old and is in 7-A class. create("His GPA is %lf.",4.96); -> His GPA is 4.96. create("His favourite subject is math!"); -> His favourite subject is math!
The part with %d and %s string was not that hard, but now I'm supposed to convert %f and %lf to string, I've tried using sprintf but I've had no luck so far, another problem is the fact that I've gotta use lists to complete the task. I've been trying to convert float to string for the past 2 hours, but I'm drawing a blank now.
int check_up(char string[]); int check_low(char string[]); void to_up(char string[]); void to_low(char string[]);
[Code] .....
When I compile this I have the following problems: warning: data definition has no type or storage class [enabled by default] in 'to_up(word)'conflicting types in 'to_up' function and to_low function warning: data definition has no type or storage class [enabled by default] into_up function error: unknown type name "word" in line 'printf("All uppercase %s. ", word):;'warning: parameter names (without types) in function declaration [enabled by default] in 'to_up(word)'and 'to_low(word)' 'note: previous declaration of "to_up" was here in function declaration of to_up function
I was wondering if one could write a function that could accept one or the other variable type.
Ex: I have 2 arrays, int** and double**, and a function
Code: void PGMWrite(double** Matrix, int Matrix_dimension){.....}
Is there any way to change the function to
Code: void PGMWrite(int** Matrix || double** Matrix, int Matrix_dimension){.....}
And then have some sort of type identifier in the function that picks the correct section via an if loop? If so how, and how would I identify in the function if the input it type double or int?
I have a function, int teleport_to_game(int,float,float); in my class. My question is should I change the int to define a function to a different type?
In my project i need to calculate the distance between two lattitude and longitude points. After compiling the code I am getting an error that "conflicting types for deg2rad". How to solve this? Seems the code looks ok. but...
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.
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.
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)
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?
