How to call a function on all variadic template args?

问题

I would like to do

template<typename... ArgTypes> void print(ArgTypes... Args)
{
   print(Args)...;
}

And have it be equivalent to this quite bulky recursive chain:

template<typename T, typename... ArgTypes> void print(const T& t, ArgTypes... Args)
{
  print(t);
  print(Args...);
}

followed by explicit single-parameter specializations for every type I'd like to print.

The "problem" with the recursive implementation is that a lot of redundant code is generated, because each recursive step results in a new function of N-1 arguments, whereas the code I'd like to have would only generate code for a single N-arg print function, and have at most N specialized print functions.

回答1:

C++17 fold expression

(f(args), ...);

If you call something that might return an object with overloaded comma operator use:

((void)f(args), ...);

Pre-C++17 solution

The typical approach here is to use a dumb list-initializer and do the expansion inside it:

{ print(Args)... }

Order of evaluation is guaranteed left-to-right in curly initialisers.

But print returns void so we need to work around that. Let's make it an int then.

{ (print(Args), 0)... }

This won't work as a statement directly, though. We need to give it a type.

using expand_type = int[];
expand_type{ (print(Args), 0)... };

This works as long as there is always one element in the Args pack. Zero-sized arrays are not valid, but we can work around that by making it always have at least one element.

expand_type{ 0, (print(Args), 0)... };

We can make this pattern reusable with a macro.

namespace so {
    using expand_type = int[];
}

#define SO_EXPAND_SIDE_EFFECTS(PATTERN) ::so::expand_type{ 0, ((PATTERN), 0)... }

// usage
SO_EXPAND_SIDE_EFFECTS(print(Args));

However, making this reusable requires a bit more attention to some details. We don't want overloaded comma operators to be used here. Comma cannot be overloaded with one of the arguments void, so let's take advantage of that.

#define SO_EXPAND_SIDE_EFFECTS(PATTERN) \
        ::so::expand_type{ 0, ((PATTERN), void(), 0)... }

If you are ~~paranoid~~ afraid of the compiler allocating large arrays of zeros for naught, you can use some other type that can be list-initialised like that but stores nothing.

namespace so {
    struct expand_type {
        template <typename... T>
        expand_type(T&&...) {}
    };
}

回答2:

C++17 fold expression:

(f(args), ...);

Keep simple things simple ;-)

If you call something that might return an object with overloaded comma operator use:

((void)f(args), ...);

回答3:

You can use even more simple and readable approach

template<typename... ArgTypes> void print(ArgTypes... Args)
{
   for (const auto& arg : {Args...})
   {
      print(arg);
   }
}

I have played with both variants on compile explorer and both gcc and clang with O3 or O2 produce exactly the same code but my variant is obviously cleaner.

来源：https://stackoverflow.com/questions/17339789/how-to-call-a-function-on-all-variadic-template-args

标签

c++

c++11

variadic-templates