Suppose, in c++ i've got a variadic template function F, and i want to convert all the arguments in some individual manner by function conv and pass all the converted arguments as a variadic list to another variadic template function G.
Then i just do the following:

template<typename T>
struct R;

template<typename T>
R<T>::Type conv(T&& val);

template<typename ... Args>
void F(Args&& ... args) {
    G(conv(std::forward<Args>(args))...);
}

Is there any way to do the same kind of thing in D? I couldn't find any unpacking operator or a library function to do so...
The problem is that i have to pass converted lits to a C-like variadic function, for example printf, so it would be like:

template<typename T>
struct R;

template<typename T>
R<T>::Type conv(T&& val);

template<typename ... Args>
void MyPrintf(const char * format, Args&& ... args) {
    printf(convFormat(format), conv(std::forward<Args>(args))...);
}

I hope, my point is clear enough.

On 07/05/2013 05:41 PM, Max Strakhov wrote:
> Suppose, in c++ i've got a variadic template function F, and i want to
> convert all the arguments in some individual manner by function conv and
> pass all the converted arguments as a variadic list to another variadic
> template function G.
> Then i just do the following:
>
> template<typename T>
> struct R;
>
> template<typename T>
> R<T>::Type conv(T&& val);
>
> template<typename ... Args>
> void F(Args&& ... args) {
>      G(conv(std::forward<Args>(args))...);
> }
>
> Is there any way to do the same kind of thing in D? I couldn't find any
> unpacking operator or a library function to do so...
> The problem is that i have to pass converted lits to a C-like variadic
> function, for example printf, so it would be like:
>
> template<typename T>
> struct R;
>
> template<typename T>
> R<T>::Type conv(T&& val);
>
> template<typename ... Args>
> void MyPrintf(const char * format, Args&& ... args) {
>      printf(convFormat(format), conv(std::forward<Args>(args))...);
> }
>
> I hope, my point is clear enough.

Here is one that works:

import core.stdc.stdio;
import std.string;

string convFormat(string s)
{
    return s ~ '\n';
}

void myPrintf(T...)(string s, T args)
{
    printf(convFormat(s).toStringz, args);
}

void main()
{
    myPrintf("format %d %f %c %s", 42, 1.5, 'a', "hello".toStringz);
}

Ideally, the caller of MyPrintf should not have to call toStringz on "hello", because myPrintf is a proper D function. However, I don't know how to manipulate args to call toStringz on string arguments.

More information is at

  http://dlang.org/template.html#TemplateTupleParameter

Ali

On 07/05/2013 06:35 PM, Ali Çehreli wrote:

> More information is at
>
>    http://dlang.org/template.html#TemplateTupleParameter

And the definitive document on templates is the following:

  https://github.com/PhilippeSigaud/D-templates-tutorial

Ali

On 07/06/13 02:41, Max Strakhov wrote:
> Suppose, in c++ i've got a variadic template function F, and i want to convert all the arguments in some individual manner by function conv and pass all the converted arguments as a variadic list to another variadic template function G.

http://forum.dlang.org/thread/yiougbmcegjlsrynlvyt@forum.dlang.org?page=2#post-mailman.1205.1371553020.13711.digitalmars-d-learn:40puremagic.com

artur

On Saturday, 6 July 2013 at 01:35:28 UTC, Ali Çehreli wrote:
> On 07/05/2013 05:41 PM, Max Strakhov wrote:
> Here is one that works:
>
> import core.stdc.stdio;
> import std.string;
>
> string convFormat(string s)
> {
>     return s ~ '\n';
> }
>
> void myPrintf(T...)(string s, T args)
> {
>     printf(convFormat(s).toStringz, args);
> }
>
> void main()
> {
>     myPrintf("format %d %f %c %s", 42, 1.5, 'a', "hello".toStringz);
> }

You only converted the format string with was not in the question. I need to convert all args by a template function, but your example is actually better than mine.
Suppose, i want my own string representation of primitive types, then in c++ i would write those two functions:
convFormat("format %d %f %c %s") -> std::string("format %s %s %s %s")
conv(42) -> std::string("int(42)")
conv(1.5)-> std::string("float(1.5)")
etc.
and the resulting printf:
template<typename ... Args>
void myPrintf(const char * format, Args&& ... args) {
   printf(convFormat(format).c_str(), conv(std::forward<Args>(args)).c_str()...);
}
And that would do the job. Can i somehow do the same thing in D?

Artur, if i use your solution like

printf("...", ForEach!(val => conv(val).c_str())(values).tuple);

Than i would get a crash, because all the tuple elements would be char*'s, pointing to already freed memory, as std::string's destructor gets called each time right after alias function exits. Ths is what c++ unpacking operator for: i actually return a packed list of std::string's, apply .c_str() to each and than unpack the list to current context.

On 07/06/13 14:21, Max Strakhov wrote:
> Artur, if i use your solution like
> 
> printf("...", ForEach!(val => conv(val).c_str())(values).tuple);
> 
> Than i would get a crash, because all the tuple elements would be char*'s, pointing to already freed memory, as std::string's destructor gets called each time right after alias function exits. Ths is what c++ unpacking operator for: i actually return a packed list of std::string's, apply .c_str() to each and than unpack the list to current context.

Can you show some simple code that exhibits the problem?

artur

Thanx for all your help, i just solved the problem with mixins like this:

string res = "";
for(i, rev v; args)
    res ~= string.format(", f(args[%d]).val()");
mixin("return G(" ~ res ~ ");");

Only without any actual looping and variables, so compiler could deal with it.

On Saturday, 6 July 2013 at 16:15:52 UTC, Max Strakhov wrote:
> Thanx for all your help, i just solved the problem with mixins like this:
>
> string res = "";
> for(i, rev v; args)
>     res ~= string.format(", f(args[%d]).val()");
> mixin("return G(" ~ res ~ ");");
>
> Only without any actual looping and variables, so compiler could deal with it.

printf("...", ForEach!(val => conv(val).c_str())(values).tuple);

...would have done exactly the same as C++:

printf("...", conv(values).c_str()...);

...does (barring it makes a temporary struct which holds one field for each of those converted const char pointers, but the compiler can optimize it away).

On Saturday, 6 July 2013 at 16:40:25 UTC, TommiT wrote:
> ...does (barring it makes a temporary struct which holds one field for each of those converted const char pointers, ...

Should be "...one field per each of those..."