sizeof operator called on the parameter pack will tell you the number of its elements at compile time. Herb Sutter mentions this in his Back to the Basics! For example, given elem_type_holder<2, some_tuple_type>, we'll get the following pseudo expansion: So the elem_type_holder<2, some_tuple_type> peels off two types from the beginning of the tuple, and sets its type to the type of the third one, which is what we need. the token preceding ## is anything other than a comma. never complain about all that, the only special case being the To fix this, we can add another variation of the function template: Here, we force all odd-numbered sequences of arguments to return false, because when only a single argument is left this version is matched. Since modern compilers are aggressively inlining code, it's likely to end up being compiled to machine code that has absolutely no function calls. This does When the macro is invoked, Lets say that you want to store them in a table. Variadic macros became a standard part of the C language with C99. In the end, the created object is not assigned to any variable, so compiler is free to optimize it away (however I didnt confirmed that it actually does that). Each parameter "e%6_|d /7k*{9~O:u
? \x9s 7 0 obj
it requires a correct base case (stop condition) implementation. Variadic template function with function in argument, Create a simple dispatcher using variadic templates, How template(General/Variadic) functionality implemented in C++ Compiler, How to fix this :warning C4840 non-portable use of class 'ATL::cstringt>>' as an argument to a variadic function.
The glue that makes it work is the comma operator, which evaluates both showImpl and 0, leaving 0 as the side effect. The v functions take a va_list parameter, the implementation of which is often done with compiler specific 'macro magic', but you are guaranteed that calling the v style function from a variadic function like this will work: This should give you the effect that you are looking for. parameters (that can be extended to variadic parameters when using arrays a function. func(a, b) will expand to func(a, b, NULL) in GNU C instead of standard. We are not basing out implementation on the recursion, so it is not needed. the format function annotation). The specialization that follows peels off() the first type from the parameter pack, and defines a member of that type named tail. And again, the result of the operator is 0. indicates the number of variadic parameters: indeed, there is no way as-is If you pass a number into a %sformat, bad things may happen and the compiler won't warn you about it [3]. func(&(struct func_params) { .sentinel_ = 0, __VA_ARGS__ }), // All of these expressions are equivalent, Benefits of sentinel without trailing NULL. I1MG /rxA5WH)q|to /oQabeajf!i^#z'5^/o+Wa-;c]r~_?y{+**&L@b^C9Q7i'cn6f}D;1*p4>Cx7\/ @>o_SH6N&%a #GUp6>|88sE-[lexgmb_VX~?w>@hQbj"C 'NYY%K/_j+V|, ? What you end up with, actually, is not unlike loop unrolling. elem_type_holder is yet another variadic class template. is still supported for backward compatibility. endobj Next: Predefined Macros, Previous: Concatenation, Up: Macros [Contents][Index]. expand to the number of times the shift happens. Below you can find a list of most interesting use cases for the expression expansion: C++17 added even more operations, that can be used with the parameter pack. How to programmatically get the CPU cache page size in C++? Generating a similar macro when the length hint is explicit is a bit the sentinel. We recommend you avoid using it except for its defined parameter without a point of reference, a marker to tell where to look, <> % In order to enable the recursion, we call the same function with the expansion of the remaining parameter pack.
It is time to ask answered 05/18/19, the wrapper function will ultimately have to take, a variable argument list so as to pass it to the. showRecursive is a function that prints its arguments, each in a separate line.
Essentials of Modern C++ Style. try to provide a better interface: These techniques provide some sugar on top of the C language to provide ambiguous. _37, _36, _35, _34, _33, _32, _31, _30, _29, _28, _27, _26, _25, _24, _23, \ Variadic templates are written just the way you'd write recursive code - you need a base case (the adder(T v) declaration above) and a general case which "recurses" [1]. char *s = va_arg(args, char*);//args the same, s bad Fortunately, there is an implicit method, that involves usage of the infamous std::initializer_list. If you are considering writing a variadic library function you should also consider making a va_list style companion available as part of the library. Variadic functions are functions (e.g. Thanks for taking your time to read it. However, as we all know, printf and its kin are not type safe. printf function family, where the parameters are type checked against If you're concerned with the performance of code that relies on variadic templates, worry not.
The same can be done with the single argument operators like incrementing, casting or extracting an address. We want it to work for any container, and we also want the user to type as little as possible, so we don't want to act on iterators. This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL). First, we'll have to define a helper type that lets us access the type of the k-th element in a tuple: NOTE: elem_type_holdertemplatetuplek****Template metaprogrammingCompile-time class generationfactorial****. This checking is done by the compiler, at compile time. Thus, we I agree this is a HACK that exploits side effects of the comma operator. Well then, without further ado, let us first quicky review how variadic The above explanation is ambiguous about the case where the only macro Its only purpose is to pass (forward) function template arguments into another function preserving all their properties like qualifiers or value semantics (lvalue, rvalue etc).
provide, at best, some techniques I have used that I think will make Again, Undefined Behavior. In pseudo-code this operation may be executed like this: If we leave only the side effects (results of the expressions), then it may look like this: This explains, why we used int as a template argument in the std::initializer_list. At best, your process will read whatever has How does make_unique know the signature of its constructor? The neat side-effect that this technique spawns is that func has But what always bothered me most with this approach is leaving something that is clearly known at compile-time, to run-time. Choose an expert and meet online. <>/XObject<>/Font<>/ProcSet[/PDF/Text/ImageB/ImageC/ImageI] >>/MediaBox[ 0 0 720 540] /Contents 4 0 R/Group<>/Tabs/S/StructParents 0>>
Understand that English isn't everyone's first language so be lenient of bad
Let us ignore for the moment all the type safety concerns, and focus on 1, 2 and 3 on separate lines. Therefore, variadic templates are often a performance optimization for variadic functions. To phrase the question another way: in general, can you wrap some arbitrary variadic function without modifying that function's definition. I will not discuss the implementation of a type-safe printf further - it has been rehashed many times already. The eprintf macro above could be written. You may use we just call va_end. 1 0 obj
It is not limited only to variadic templates, but in my opinion it brings the most benefits in this context. 4 0 obj
You first need to look at the documentation for SDL_Log to see what parameters it expects. Hm yes, but none of the solutions seem to work. There is one last very important technique called perfect forwarding. Usually, it is exploited in the highly customizable libraries, where client can specify its own callback functions. char *s = va_arg(argptr, char*); //s correct
Furthermore, if you left the variable argument empty, you would have For people used to C++98 style or earlier this seems like a cool trick. it could continue to run as if nothing happened no need to tell how bad Not directly, however it is common (and you will find almost universally the case in the standard library) for variadic functions to come in pairs with a varargs style alternative function. Programmatically find the number of cores on a machine. used, then the comma before the ## will be deleted. 44, 43, 42, 41, 40, 39, 38, 37, 36, 35, 34, 33, 32, 31, 30, 29, 28, 27, 26,\ This is a recursive definition that stops when there are no more types to peel off, and the base of the hierarchy is an empty tuple. func(a, b, , NULL) in ANSI C (making one of the variadic parameters all the trailing numbers to the right in ARG_LENGTH_, making Count In standard C, you could not omit the comma separating the If the variadic function takes a variable number of arguments. <>
the # and ## operators to stringize the variable argument It takes a number k and the tuple type we're interested in as template parameters. Note how the general adderis defined - the first argument is peeled off() the template parameter pack into type T (and accordingly, argument first). endobj
Do you need your, CodeProject,
The syntax for defining the macro is similar to that of va_arg not only read, but move list pointer. Yes, I know. Unlike function types that specify their arguments at compile time, given just a generic type T we don't know which constructor(s) it has and how many arguments the constructor accepts. adder will accept any number of arguments, and will compile properly as long as it can apply the + operator to them. This sequence is, in practice, fairly short (variadic calls with more than 5-6 arguments are rare). Since we cant create an initializer list out of the parameter pack (which could potentially have multiple types), we use a list of 0s preceded with a demanded function.
This effect is sometimes can only be last in a parameter list, they are pushed first <> replacement list of a variadic macro. NOTE: Basic exampletemplate+overloadcompilerresolutionC-family-language\C++\Guide\Implementation-of-polymorphism\index.mdOverload + templateresolution, base case recursion ending condition . after the format string. It's different from the ones already shown in the article, because it doesn't really use the traditional recursive approach of implementing variadic templates. // Implement << for pairs: this is needed to print out mappings where range. [Update: there seems to be some confusion based on the answers that have been supplied so far. For Free, 2005 - 2022 Wyzant, Inc, a division of IXL Learning - All Rights Reserved |, Metric Prefixes and Their Origins and Use. %PDF-1.7 Note that pair_comparer forces both members of the compared pair to be of the exact same type. variadic functions more usable. Note that this is a compile-time template metaprogramming construct - it acts on constants and types, not on runtime objects. for example, you pass 2 instead of 2.; subtle, but deadly). Simple indeed, but incredibly unsafe. e.g. be used as a macro name, macro argument name, or within a different type The argument of its constructor is particularly interesting: This is a pack expansion, but used on the comma operator expression. Some of the most popular standard functions and classes that use perfect forwarding are: Even Bjarne Stroustrup was joking (or not), that he cant write perfect forwarding with confidence without checking some details first. So with each call, the parameter pack gets shorter by one parameter. Finally, in the last round showRecursive will be called with an empty pack. Rather, it uses them to express the "any template parameters can go here" concept. have this expansion: The variable argument is completely macro-expanded before it is inserted sizeof() will tell you: This technique is pretty straightforward. A very important example of this is the std::make_unique function, available in the standard library since C++14. We want to be able to use it as follows: FooType is an arbitrary type and can be constructed in arbitrary ways. x[k1T;#i@!@,C%1wvMo>e4g7oE;bry{O$6sk5`yo c7Sg#k>[3)3:0(^1J:kfw%H(D~ogp|PQ^#$6N$8]%o]>z}^XdRj[_a,^%z]EB.r1!yIlVK^08!ftw]vXl|5g0x@#p Q0oF[vd : Note: It is worth noticing that in the first case, decltype(t) is the same as T, but not in the second. Thus each instantiation of the template will assign its leading argument to the first variable and leave the rest as the remaining pack. try to distinguish whether no argument at all is an empty argument or For example: Returns true. If is manipulated, must restore from saved value. But lets not forget, that templates were never intended to be used this way. In addition to being type unsafe (all type resolution has to be done explicitly with casts in va_arg, at runtime), it's also tricky to get right. If you don't have a function analogous to vfprintf that takes a va_list instead of a variable number of arguments, you can't do it. of macro. A link to the app was sent to your phone. ARG_LENGTH__(__VA_ARGS__)) // point types to use approximate comparison. forward: The only difference between the ANSI and GNU version is that Here's how make_unique is typically implemented: Ignore the && syntax and std::forward for now; I will cover them in a future article. Otherwise the comma is dropped as an extension to the standard. Then the second unpacked argument is passed to the constructor of the std::initializer_list in the same manner. the order and size of your parameters (this can cause horrible results if, In the case of printf, when the implementation reaches a new formatting directive it can actually assert the type of the argument passed. #endif, # define ARG_LENGTH__() ARG_LENGTH_(,##__VA_ARGS__, \ We are not, in 99% of the time, designing functions la printf, where a python-like interface for keyword arguments: Of course, one must also consider that this technique provides default 5 0 obj printf are blessed with parameters that convey both a meaning and The recursion itself happens in the call adder(args). increments argptr so when you call Pandas how to find column contains a certain value, Recommended way to install multiple Python versions on Ubuntu 20.04, Build super fast web scraper with Python x100 than BeautifulSoup, How to convert a SQL query result to a Pandas DataFrame in Python, How to write a Pandas DataFrame to a .csv file in Python. endobj Today I want to show you a few techniques of variadic templates, that can be found in many codebases. stream To access the variadic arguments from the function body, the following library facilities are provided: // note automatic conversion to integral type, https://en.cppreference.com/mwiki/index.php?title=cpp/utility/variadic&oldid=105915, enables access to variadic function arguments, accesses the next variadic function argument, makes a copy of the variadic function arguments, ends traversal of the variadic function arguments, holds the information needed by va_start, va_arg, va_end, and va_copy. But that is not what you are trying to pass. It is also very rarely found in a typical business code, thus I will not cover its details here. spelling and grammar. It may not It would make sense to specialize this function for floating. We first have to define a macro to count the variadic parameters The #if __STRICT_ANSI__ block is here to have consistent results with sentinel_ is here to ensure that func called without parameters still int printf(const char* format, );. I have mentioned printf in the beginning of the article, as an example of a variadic function that doesn't use templates. right, Undefined Behavior. Variadic templates for the rescue: What this says is - ContainerType is a template template parameter with any amount of template parameters itself. So, the struct definition above lets us create tuples, but there's not much else we can do with them yet. Normal function pointers for instance don't do the trick here, as std::function doesn't seems to accept (). Here is an example: This kind of macro is called variadic. Unfortunately there is no other simple way to do it if that is the only SDL_Log function that is defined. endobj Contrary to your comment on that line, CPP retains the comma when conforming to a specific C (Note that this difference being that types in those are checked. Variadic functions Part 3: techniques of variadic templates, Variadic functions Part 1: va_args and friends, Variadic functions Part 2: C++11 variadic templates, Non-recursive argument evaluation with std::initializer_list, Back to the Basics! But if we change the second argument to just 1, this won't compile since a double and int are not the same type. email is in use. The same problem would occur for a set, which has 3 template arguments. Yes, when we write a variadic function we don't know all the ways it's going to be used. has some drawbacks. Thats flexible: you had to supply at least one argument after the format The example shown above is very basic - template arguments are peeled off one by one until the base case is hit. the number of expected variadic arguments (i.e. +1 (416) 849-8900. Let's dive in, by implementing a function that adds all of its arguments together: And here are a couple of ways we could call it: compilerbase conditionT adder(T v). A macro can be declared to accept a variable number of arguments much as parameter is a variable arguments parameter, as it is meaningless to STL would be an excellent example of this. For the full code that you can play with and compile on your own: variadic-tuple.cpp. been pushed before 3, try do dereference it, and will crash, but worse, Using Clang's layout dump feature, we can verify this: Indeed, the size of the data structure and the internal layout of members is as expected. va_arg is not typesafe, meaning that you have to care about 6 0 obj Armed with this, we can implement get: Here, enable_if is used to select between two template overloads of get - one for when k is zero, and one for the general case which peels off the first type and recurses, as usual with variadic function templates. See http://c-faq.com/varargs/handoff.html. You can also subscribe to my newsletter to get latest news about my content. The prime example of this is a tuple class, and here I want to show how to construct one [4]. We start with the base case - the definition of a class template named tuple, which is empty. list in a va_list, with n as a point of reference. You cannot use __VA_ARGS__ and this How big should it be? Compared to the C-style variadic functions, this is a marked win, because C-style variadic arguments have to be resolved at runtime. C89, you would have to replace the compound literal with a local variable If you write. purpose. Order of items in classes: Fields, Properties, Constructors, Methods. You may write an argument name immediately As there's no actual recursion involved, all we have is a sequence of function calls pre-generated at compile-time. std::printf) which take a variable number of arguments. As we already know, we have to unpack the first argument from the parameter pack by explicitly declaring it in the function parameter list. ], a Question Visit Microsoft Q&A to post new questions. It will still produce a warning if -Wmissing-field-initializer is
The glue that makes it work is the comma operator, which evaluates both showImpl and 0, leaving 0 as the side effect. The v functions take a va_list parameter, the implementation of which is often done with compiler specific 'macro magic', but you are guaranteed that calling the v style function from a variadic function like this will work: This should give you the effect that you are looking for. parameters (that can be extended to variadic parameters when using arrays a function. func(a, b) will expand to func(a, b, NULL) in GNU C instead of standard. We are not basing out implementation on the recursion, so it is not needed. the format function annotation). The specialization that follows peels off() the first type from the parameter pack, and defines a member of that type named tail. And again, the result of the operator is 0. indicates the number of variadic parameters: indeed, there is no way as-is If you pass a number into a %sformat, bad things may happen and the compiler won't warn you about it [3]. func(&(struct func_params) { .sentinel_ = 0, __VA_ARGS__ }), // All of these expressions are equivalent, Benefits of sentinel without trailing NULL. I1MG /rxA5WH)q|to /oQabeajf!i^#z'5^/o+Wa-;c]r~_?y{+**&L@b^C9Q7i'cn6f}D;1*p4>Cx7\/ @>o_SH6N&%a #GUp6>|88sE-[lexgmb_VX~?w>@hQbj"C 'NYY%K/_j+V|, ? What you end up with, actually, is not unlike loop unrolling. elem_type_holder is yet another variadic class template. is still supported for backward compatibility. endobj Next: Predefined Macros, Previous: Concatenation, Up: Macros [Contents][Index]. expand to the number of times the shift happens. Below you can find a list of most interesting use cases for the expression expansion: C++17 added even more operations, that can be used with the parameter pack. How to programmatically get the CPU cache page size in C++? Generating a similar macro when the length hint is explicit is a bit the sentinel. We recommend you avoid using it except for its defined parameter without a point of reference, a marker to tell where to look, <> % In order to enable the recursion, we call the same function with the expansion of the remaining parameter pack.
It is time to ask answered 05/18/19, the wrapper function will ultimately have to take, a variable argument list so as to pass it to the. showRecursive is a function that prints its arguments, each in a separate line.
Essentials of Modern C++ Style. try to provide a better interface: These techniques provide some sugar on top of the C language to provide ambiguous. _37, _36, _35, _34, _33, _32, _31, _30, _29, _28, _27, _26, _25, _24, _23, \ Variadic templates are written just the way you'd write recursive code - you need a base case (the adder(T v) declaration above) and a general case which "recurses" [1]. char *s = va_arg(args, char*);//args the same, s bad Fortunately, there is an implicit method, that involves usage of the infamous std::initializer_list. If you are considering writing a variadic library function you should also consider making a va_list style companion available as part of the library. Variadic functions are functions (e.g. Thanks for taking your time to read it. However, as we all know, printf and its kin are not type safe. printf function family, where the parameters are type checked against If you're concerned with the performance of code that relies on variadic templates, worry not.
The same can be done with the single argument operators like incrementing, casting or extracting an address. We want it to work for any container, and we also want the user to type as little as possible, so we don't want to act on iterators. This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL). First, we'll have to define a helper type that lets us access the type of the k-th element in a tuple: NOTE: elem_type_holdertemplate
This effect is sometimes can only be last in a parameter list, they are pushed first <> replacement list of a variadic macro. NOTE: Basic exampletemplate+overloadcompilerresolutionC-family-language\C++\Guide\Implementation-of-polymorphism\index.mdOverload + templateresolution, base case recursion ending condition . after the format string. It's different from the ones already shown in the article, because it doesn't really use the traditional recursive approach of implementing variadic templates. // Implement << for pairs: this is needed to print out mappings where range. [Update: there seems to be some confusion based on the answers that have been supplied so far. For Free, 2005 - 2022 Wyzant, Inc, a division of IXL Learning - All Rights Reserved |, Metric Prefixes and Their Origins and Use. %PDF-1.7 Note that pair_comparer forces both members of the compared pair to be of the exact same type. variadic functions more usable. Note that this is a compile-time template metaprogramming construct - it acts on constants and types, not on runtime objects. for example, you pass 2 instead of 2.; subtle, but deadly). Simple indeed, but incredibly unsafe. e.g. be used as a macro name, macro argument name, or within a different type The argument of its constructor is particularly interesting: This is a pack expansion, but used on the comma operator expression. Some of the most popular standard functions and classes that use perfect forwarding are: Even Bjarne Stroustrup was joking (or not), that he cant write perfect forwarding with confidence without checking some details first. So with each call, the parameter pack gets shorter by one parameter. Finally, in the last round showRecursive will be called with an empty pack. Rather, it uses them to express the "any template parameters can go here" concept. have this expansion: The variable argument is completely macro-expanded before it is inserted sizeof() will tell you: This technique is pretty straightforward. A very important example of this is the std::make_unique function, available in the standard library since C++14. We want to be able to use it as follows: FooType is an arbitrary type and can be constructed in arbitrary ways. x[k1T;#i@!@,C%1wvMo>e4g7oE;bry{O$6sk5`yo c7Sg#k>[3)3:0(^1J:kfw%H(D~ogp|PQ^#$6N$8]%o]>z}^XdRj[_a,^%z]EB.r1!yIlVK^08!ftw]vXl|5g0x@#p Q0oF[vd : Note: It is worth noticing that in the first case, decltype(t) is the same as T, but not in the second. Thus each instantiation of the template will assign its leading argument to the first variable and leave the rest as the remaining pack. try to distinguish whether no argument at all is an empty argument or For example: Returns true. If is manipulated, must restore from saved value. But lets not forget, that templates were never intended to be used this way. In addition to being type unsafe (all type resolution has to be done explicitly with casts in va_arg, at runtime), it's also tricky to get right. If you don't have a function analogous to vfprintf that takes a va_list instead of a variable number of arguments, you can't do it. of macro. A link to the app was sent to your phone. ARG_LENGTH__(__VA_ARGS__)) // point types to use approximate comparison. forward: The only difference between the ANSI and GNU version is that Here's how make_unique is typically implemented: Ignore the && syntax and std::forward for now; I will cover them in a future article. Otherwise the comma is dropped as an extension to the standard. Then the second unpacked argument is passed to the constructor of the std::initializer_list in the same manner. the order and size of your parameters (this can cause horrible results if, In the case of printf, when the implementation reaches a new formatting directive it can actually assert the type of the argument passed. #endif, # define ARG_LENGTH__() ARG_LENGTH_(,##__VA_ARGS__, \ We are not, in 99% of the time, designing functions la printf, where a python-like interface for keyword arguments: Of course, one must also consider that this technique provides default 5 0 obj printf are blessed with parameters that convey both a meaning and The recursion itself happens in the call adder(args). increments argptr so when you call Pandas how to find column contains a certain value, Recommended way to install multiple Python versions on Ubuntu 20.04, Build super fast web scraper with Python x100 than BeautifulSoup, How to convert a SQL query result to a Pandas DataFrame in Python, How to write a Pandas DataFrame to a .csv file in Python. endobj Today I want to show you a few techniques of variadic templates, that can be found in many codebases. stream To access the variadic arguments from the function body, the following library facilities are provided: // note automatic conversion to integral type, https://en.cppreference.com/mwiki/index.php?title=cpp/utility/variadic&oldid=105915, enables access to variadic function arguments, accesses the next variadic function argument, makes a copy of the variadic function arguments, ends traversal of the variadic function arguments, holds the information needed by va_start, va_arg, va_end, and va_copy. But that is not what you are trying to pass. It is also very rarely found in a typical business code, thus I will not cover its details here. spelling and grammar. It may not It would make sense to specialize this function for floating. We first have to define a macro to count the variadic parameters The #if __STRICT_ANSI__ block is here to have consistent results with sentinel_ is here to ensure that func called without parameters still int printf(const char* format, );. I have mentioned printf in the beginning of the article, as an example of a variadic function that doesn't use templates. right, Undefined Behavior. Variadic templates for the rescue: What this says is - ContainerType is a template template parameter with any amount of template parameters itself. So, the struct definition above lets us create tuples, but there's not much else we can do with them yet. Normal function pointers for instance don't do the trick here, as std::function doesn't seems to accept (). Here is an example: This kind of macro is called variadic. Unfortunately there is no other simple way to do it if that is the only SDL_Log function that is defined. endobj Contrary to your comment on that line, CPP retains the comma when conforming to a specific C (Note that this difference being that types in those are checked. Variadic functions Part 3: techniques of variadic templates, Variadic functions Part 1: va_args and friends, Variadic functions Part 2: C++11 variadic templates, Non-recursive argument evaluation with std::initializer_list, Back to the Basics! But if we change the second argument to just 1, this won't compile since a double and int are not the same type. email is in use. The same problem would occur for a set, which has 3 template arguments. Yes, when we write a variadic function we don't know all the ways it's going to be used. has some drawbacks. Thats flexible: you had to supply at least one argument after the format The example shown above is very basic - template arguments are peeled off one by one until the base case is hit. the number of expected variadic arguments (i.e. +1 (416) 849-8900. Let's dive in, by implementing a function that adds all of its arguments together: And here are a couple of ways we could call it: compilerbase conditionT adder(T v). A macro can be declared to accept a variable number of arguments much as parameter is a variable arguments parameter, as it is meaningless to STL would be an excellent example of this. For the full code that you can play with and compile on your own: variadic-tuple.cpp. been pushed before 3, try do dereference it, and will crash, but worse, Using Clang's layout dump feature, we can verify this: Indeed, the size of the data structure and the internal layout of members is as expected. va_arg is not typesafe, meaning that you have to care about 6 0 obj Armed with this, we can implement get: Here, enable_if is used to select between two template overloads of get - one for when k is zero, and one for the general case which peels off the first type and recurses, as usual with variadic function templates. See http://c-faq.com/varargs/handoff.html. You can also subscribe to my newsletter to get latest news about my content. The prime example of this is a tuple class, and here I want to show how to construct one [4]. We start with the base case - the definition of a class template named tuple, which is empty. list in a va_list, with n as a point of reference. You cannot use __VA_ARGS__ and this How big should it be? Compared to the C-style variadic functions, this is a marked win, because C-style variadic arguments have to be resolved at runtime. C89, you would have to replace the compound literal with a local variable If you write. purpose. Order of items in classes: Fields, Properties, Constructors, Methods. You may write an argument name immediately As there's no actual recursion involved, all we have is a sequence of function calls pre-generated at compile-time. std::printf) which take a variable number of arguments. As we already know, we have to unpack the first argument from the parameter pack by explicitly declaring it in the function parameter list. ], a Question Visit Microsoft Q&A to post new questions. It will still produce a warning if -Wmissing-field-initializer is