有没有办法在 VS2012 中编写 make_unique() ?

2021-12-13 00:00:00 templates c++ c++11 unique-ptr variadic-templates

Herb Sutter 提出了一个简单的 make_unique() 实现:http://herbsutter.com/gotw/_102/

Herb Sutter propose a simple implementation of make_unique() there: http://herbsutter.com/gotw/_102/

这是:

template<typename T, typename ...Args>
std::unique_ptr<T> make_unique( Args&& ...args )
{
    return std::unique_ptr<T>( new T( std::forward<Args>(args)... ) );
}

我的问题是可变参数模板还不是 VS2012 的一部分，所以我不能按原样使用此代码.

My problem is that variadic templates are not yet part of VS2012, so I can't use this code as is.

在 VS2012 中是否有一种可维护的方法来编写它，而不涉及使用不同的 args 计数复制粘贴相同的函数?

推荐答案

您可以使用 Boost.Preprocessor 来生成不同的参数计数，但我真的没有看到这样做的好处.简单地做一次繁重的工作，把它塞进一个标题中就可以了.您可以节省自己的编译时间并拥有 make_unique.

You could use Boost.Preprocessor to generate the different parameter counts, but I really don't see the advantage of that. Simply do the grunt job once, stuff it in a header and be done. You're saving yourself compile time and have your make_unique.

这里是我的 make_unique.h 标头的复制粘贴模拟最多 5 个参数的可变参数模板.

Here's a copy-paste of my make_unique.h header that simulates variadic templates for up to 5 arguments.

由于 OP 似乎不喜欢复制粘贴工作，这里是生成上述内容的 Boost.Preprocessor 代码:

Since OP seems to not like copy-paste work, here's the Boost.Preprocessor code to generate the above:

首先，创建一个包含多次模板头的主头(Boost.Preprocessor 迭代代码公然从这个答案中窃取)):

First, make a main header that includes the template header multiple times (Boost.Preprocessor iteration code blatantly stolen from this answer):

// make_unique.h
#include <memory>
#include <utility>
#include <boost/preprocessor.hpp>

#ifndef MAKE_UNIQUE_NUM_ARGS
// allow this to be changed to a higher number if needed,
// ten is a good default number
#define MAKE_UNIQUE_NUM_ARGS 10
#endif

#if MAKE_UNIQUE_NUM_ARGS < 0
// but don't be stupid with it
#error Invalid MAKE_UNIQUE_NUM_ARGS value.
#endif

/* optional, see above for premade version
// include premade functions, to avoid the costly iteration
#include "detail/blah_premade.hpp

// generate classes if needed
#if MAKE_UNIQUE_NUM_ARGS > MAKE_UNIQUE_NUM_PREMADE
*/
#define BOOST_PP_ITERATION_LIMITS (0, MAKE_UNIQUE_NUM_ARGS)
#define BOOST_PP_FILENAME_1 "make_unique_template.h"
#include BOOST_PP_ITERATE()
//#endif

现在制作一个模板标题，根据MAKE_UNIQUE_NUM_ARGS的值一次又一次地包含并以不同的方式扩展:

And now make a template header that gets included again and again and expands differently depending on the value of MAKE_UNIQUE_NUM_ARGS:

// make_unique_template.h
// note: no include guard

#define N BOOST_PP_ITERATION()    

#define MAKE_UNIQUE_TEMPLATE_PARMS 
  BOOST_PP_ENUM_PARAMS(N, typename A)

#define MAKE_UNIQUE_FUNCTION_PARM(J,I,D) 
  BOOST_PP_CAT(A,I)&& BOOST_PP_CAT(a,I)

#define MAKE_UNIQUE_FUNCTION_PARMS 
  BOOST_PP_ENUM(N, MAKE_UNIQUE_FUNCTION_PARM, BOOST_PP_EMPTY)

#define MAKE_UNIQUE_ARG(J,I,D) 
  std::forward<BOOST_PP_CAT(A,I)>(BOOST_PP_CAT(a,I))

#define MAKE_UNIQUE_ARGS 
  BOOST_PP_ENUM(N, MAKE_UNIQUE_ARG, BOOST_PP_EMPTY)

template<class T BOOST_PP_COMMA_IF(N) MAKE_UNIQUE_TEMPLATE_PARMS>
std::unique_ptr<T> make_unique(MAKE_UNIQUE_FUNCTION_PARMS){
  return std::unique_ptr<T>(new T(MAKE_UNIQUE_ARGS));
}

// clean up
#undef MAKE_UNIQUE_TEMPLATE_PARMS
#undef MAKE_UNIQUE_FUNCTION_PARM
#undef MAKE_UNIQUE_FUNCTION_PARMS
#undef MAKE_UNIQUE_ARG
#undef MAKE_UNIQUE_ARGS
#undef N

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